git.txt: add list of guides

Not all man5/man7 guides are mentioned in the 'git(1)' documentation,
which makes the missing ones somewhat hard to find.

Add a list of the guides to git(1) by leveraging the existing
`Documentation/cmd-list.perl` script to generate a file `cmds-guide.txt`
which gets included in git.txt.

Also, do not hard-code the manual section '1'. Instead, use a regex so
that the manual section is discovered from the first line of each
`git*.txt` file.

This addition was hinted at in 1b81d8c (help: use command-list.txt
for the source of guides, 2018-05-20).

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: phil-blain

Documentation/Makefile

@@ -295,6 +295,7 @@ cmds_txt = cmds-ancillaryinterrogators.txt \
 	cmds-plumbingmanipulators.txt \
 	cmds-synchingrepositories.txt \
 	cmds-synchelpers.txt \
+	cmds-guide.txt \
 	cmds-purehelpers.txt \
 	cmds-foreignscminterface.txt
 

Documentation/cmd-list.perl

@@ -6,9 +6,14 @@ sub format_one {
 	my ($out, $nameattr) = @_;
 	my ($name, $attr) = @$nameattr;
 	my ($state, $description);
+	my $mansection;
 	$state = 0;
 	open I, '<', "$name.txt" or die "No such file $name.txt";
 	while (<I>) {
+		if (/^git[a-z0-9-]*\(([0-9])\)$/) {
+			$mansection = $1;
+			next;
+		}
 		if (/^NAME$/) {
 			$state = 1;
 			next;
@@ -27,7 +32,7 @@ sub format_one {
 		die "No description found in $name.txt";
 	}
 	if (my ($verify_name, $text) = ($description =~ /^($name) - (.*)/)) {
-		print $out "linkgit:$name\[1\]::\n\t";
+		print $out "linkgit:$name\[$mansection\]::\n\t";
 		if ($attr =~ / deprecated /) {
 			print $out "(deprecated) ";
 		}

Documentation/git.txt

@@ -304,6 +304,13 @@ users typically do not use them directly.
 
 include::cmds-purehelpers.txt[]
 
+Guides
+------
+
+The following documentation pages are guides about Git concepts.
+
+include::cmds-guide.txt[]
+
 
 Configuration Mechanism
 -----------------------