README.md: Start with installing git if not installed already

Author: Matthias Koeppe

README.md

@@ -150,7 +150,22 @@ in the Installation Guide.
       capitalization when changing into :envvar:`SAGE_ROOT` can lead to build
       errors for dependencies requiring exact capitalization in path names.
 
-2.  Clone the sources.
+2.  Clone the sources with `git`:
+
+    - To check that `git` is available, open a terminal and enter
+      the following command at the shell prompt (`$`):
+
+            $ git --version
+            git version 2.42.0
+
+      The exact version does not matter, but if this command gives an error,
+      install `git` using your package manager, using one of these commands:
+
+            $ sudo pacman -S git                          # on Arch Linux
+            $ sudo apt-get update && apt-get install git  # on Debian/Ubuntu
+            $ sudo yum install git                        # on Fedora/Redhat/CentOS
+            $ sudo zypper install git                     # on openSUSE
+            $ sudo xbps-install git                       # on Void Linux
 
     - Create the directory where `SAGE_ROOT` should be established:
 
@@ -163,16 +178,18 @@ in the Installation Guide.
                         --origin upstream --branch develop --tags \
                         https://github.com/sagemath/sage.git
 
+      This command obtains the most recent development release.
+      Replace `--branch develop` by `--branch master` to select
+      the most recent stable release instead.
+
       This will create the subdirectory `~/sage/sage`. (See the section
       [Setting up git](https://doc.sagemath.org/html/en/developer/git_setup.html)
       and the following sections in the Sage Developer's Guide
       for more information.)
 
-      Change into it and pick the branch you need, typically
-      the latest development branch:
+    - Change into the created subdirectory:
 
             $ cd sage
-            $ git checkout develop
 
     - [Windows] The Sage source tree contains symbolic links, and the
       build will not work if Windows line endings rather than UNIX
@@ -181,7 +198,16 @@ in the Installation Guide.
       Therefore it is recommended (but not necessary) to use the
       WSL version of `git`.
 
-3.  [Linux, WSL] Install the required minimal build prerequisites.
+3.  Install system packages.
+
+    Either refer for this to the [section on installation from
+    source](https://doc.sagemath.org/html/en/installation/source.html) in the
+    Sage Installation Manual for compilations of system packages
+    that you can install. When done, skip to step 7 (bootstrapping).
+
+    Alternatively, follow the more fine-grained approach below.
+
+4.  [Linux, WSL] Install the required minimal build prerequisites:
 
     - Compilers: `gcc`, `gfortran`, `g++` (GCC versions from 8.4.0 to 13.x
       and recent versions of Clang (LLVM) are supported).
@@ -190,12 +216,12 @@ in the Installation Guide.
       for a discussion of suitable compilers.
 
     - Build tools: GNU `make`, GNU `m4`, `perl` (including
-      ``ExtUtils::MakeMaker``), `ranlib`, `git`, `tar`, `bc`.
+      `ExtUtils::MakeMaker`), `ranlib`, `git`, `tar`, `bc`.
       See [build/pkgs/_prereq/SPKG.rst](build/pkgs/_prereq/SPKG.rst) for
       more details.
 
     - Python 3.4 or later, or Python 2.7, a full installation including
-      `urllib`; but ideally version 3.9.x, 3.10.x, or 3.11.x, which
+      `urllib`; but ideally version 3.9.x, 3.10.x, 3.11.x, 3.12.x, which
       will avoid having to build Sage's own copy of Python 3.
       See [build/pkgs/python3/SPKG.rst](build/pkgs/python3/SPKG.rst)
       for more details.
@@ -214,22 +240,25 @@ in the Installation Guide.
     [void.txt](build/pkgs/_prereq/distros/void.txt), or visit
     https://doc.sagemath.org/html/en/reference/spkg/_prereq.html#spkg-prereq
 
-4.  [Git] If you plan to do Sage development or otherwise work with ticket branches
-    and not only releases, install the bootstrapping prerequisites. See the
-    files in the folder
+5.  Optional: It is recommended that you have both LaTeX and
+    the ImageMagick tools (e.g. the "convert" command) installed
+    since some plotting functionality benefits from them.
+
+6.  [Development] If you plan to do Sage development or otherwise work with
+    ticket branches and not only releases, install the bootstrapping
+    prerequisites. See the files in the folder
     [build/pkgs/_bootstrap/distros](build/pkgs/_bootstrap/distros), or
     visit
     https://doc.sagemath.org/html/en/reference/spkg/_bootstrap.html#spkg-bootstrap
 
-5.  [Git] If you cloned the Sage repository using `git`, bootstrap the
-    source tree using the following command:
+7.  Bootstrap the source tree using the following command:
 
         $ make configure
 
-    (If the bootstrapping prerequisites are not installed, this command will
-    download a package providing pre-built bootstrap output instead.)
+    (If the bootstrapping prerequisites are not installed, this command
+    will download a package providing pre-built bootstrap output instead.)
 
-6.  Sanitize the build environment. Use the command
+8.  Sanitize the build environment. Use the command
 
         $ env
 
@@ -261,7 +290,7 @@ in the Installation Guide.
       can also add it to your shell profile so that it gets run
       automatically in all future sessions.)
 
-7.  Optionally, decide on the installation prefix (`SAGE_LOCAL`):
+9.  Optionally, decide on the installation prefix (`SAGE_LOCAL`):
 
     - Traditionally, and by default, Sage is installed into the
       subdirectory hierarchy rooted at `SAGE_ROOT/local/`.
@@ -279,15 +308,11 @@ in the Installation Guide.
       installs (`make install` is a no-op).  Therefore the
       installation hierarchy must be writable by the user.
 
-    - See the installation manual for options if you want to
+    - See the Sage Installation Manual for options if you want to
       install into shared locations such as `/usr/local/`.
       Do not attempt to build Sage as `root`.
 
-8.  Optional: It is recommended that you have both LaTeX and
-    the ImageMagick tools (e.g. the "convert" command) installed
-    since some plotting functionality benefits from them.
-
-9.  Optionally, review the configuration options, which includes
+10. Optionally, review the configuration options, which includes
     many optional packages:
 
         $ ./configure --help
@@ -305,7 +330,7 @@ in the Installation Guide.
       a great speedup when switching between different branches, at the
       expense of disk space use.
 
-10. Optional, but highly recommended: Set some environment variables to
+11. Optional, but highly recommended: Set some environment variables to
     customize the build.
 
     For example, the `MAKE` environment variable controls whether to
@@ -327,7 +352,7 @@ in the Installation Guide.
     building Sage, see [the installation
     guide](https://doc.sagemath.org/html/en/installation/source.html#environment-variables).
 
-11. Type `./configure`, followed by any options that you wish to use.
+12. Type `./configure`, followed by any options that you wish to use.
     For example, to build Sage with `gf2x` package supplied by Sage,
     use `./configure --with-system-gf2x=no`.
 
@@ -347,60 +372,60 @@ in the Installation Guide.
     available; only the most recent releases of your distribution will
     have all of these recommended packages.
 
-12. Optional: If you choose to install the additional system packages,
+13. Optional: If you choose to install the additional system packages,
     a re-run of `./configure` will test whether the versions installed
     are usable for Sage; if they are, this will reduce the compilation
     time and disk space needed by Sage. The usage of packages may be
     adjusted by `./configure` parameters (check again the output of
     `./configure --help`).
 
-13. Type `make`.  That's it! Everything is automatic and
+14. Type `make`.  That's it! Everything is automatic and
     non-interactive.
 
     If you followed the above instructions, in particular regarding the
     installation of system packages recommended by the output of
-    `./configure` (step 10), and regarding the parallel build (step 9),
+    `./configure` (step 11), and regarding the parallel build (step 10),
     building Sage takes less than one hour on a modern computer.
     (Otherwise, it can take much longer.)
 
     The build should work fine on all fully supported platforms. If it
     does not, we want to know!
 
-14. Type `./sage` to try it out. In Sage, try for example `2 + 2`,
+15. Type `./sage` to try it out. In Sage, try for example `2 + 2`,
     `plot(x^2)`, `plot3d(lambda x, y: x*y, (-1, 1), (-1, 1))`
     to test a simple computation and plotting in 2D and 3D.
     Type <kbd>Ctrl</kbd>+<kbd>D</kbd> or `quit` to quit Sage.
 
-15. Optional: Type `make ptestlong` to test all examples in the documentation
+16. Optional: Type `make ptestlong` to test all examples in the documentation
     (over 200,000 lines of input!) -- this takes from 10 minutes to
     several hours. Don't get too disturbed if there are 2 to 3 failures,
     but always feel free to email the section of `logs/ptestlong.log` that
     contains errors to the [sage-support mailing list](https://groups.google.com/group/sage-support).
     If there are numerous failures, there was a serious problem with your build.
 
-16. The HTML version of the [documentation](https://doc.sagemath.org/html/en/index.html)
+17. The HTML version of the [documentation](https://doc.sagemath.org/html/en/index.html)
     is built during the compilation process of Sage and resides in the directory
     `local/share/doc/sage/html/`. You may want to bookmark it in your browser.
 
-17. Optional: If you want to build the PDF version of the documentation,
+18. Optional: If you want to build the PDF version of the documentation,
     run `make doc-pdf` (this requires LaTeX to be installed).
 
-18. Optional: Install optional packages of interest to you:
+19. Optional: Install optional packages of interest to you:
     get a list by typing  `./sage --optional` or by visiting the
     [packages documentation page](https://doc.sagemath.org/html/en/reference/spkg/).
 
-19. Optional: Create a symlink to the installed `sage` script in a
-    directory in your `PATH`, for example ``/usr/local``. This will
+20. Optional: Create a symlink to the installed `sage` script in a
+    directory in your `PATH`, for example `/usr/local`. This will
     allow you to start Sage by typing `sage` from anywhere rather than
     having to either type the full path or navigate to the Sage
     directory and type `./sage`. This can be done by running:
 
         $ sudo ln -s $(./sage -sh -c 'ls $SAGE_ROOT/venv/bin/sage') /usr/local/bin
 
-20. Optional: Set up SageMath as a Jupyter kernel in an existing Jupyter notebook
+21. Optional: Set up SageMath as a Jupyter kernel in an existing Jupyter notebook
     or JupyterLab installation, as described in [section
     "Launching SageMath"](https://doc.sagemath.org/html/en/installation/launching.html)
-    in the installation manual.
+    in the Sage Installation Manual.
 
 Alternative Installation using PyPI
 ---------------