completion: clarify support for short options and arguments

The list of supported completions in the header of the file was
mostly written a long time ago when Shawn added the initial version
of this script in 2006.  The list explicitly states that we complete
"common --long-options", which implies that we do not complete
not-so-common ones and single letter options (this text dates back
to May 2007).

Update the description to explicitly state that single-letter
options are not completed.  Also, document that arguments to options
are completed, even for single-letter options (e.g., "git -c <TAB>"
offers configuration variables).

The reason why we do not complete single-letter options is because
it does not seem to help all that much to learn that the command
takes -c, -d, -e options when "git foo -<TAB>" offers these three,
unlike long options that is easier to guess what they are about.

Because this rationale is primarily for our developers, let's leave
it out of the completion script itself, whose messages are entirely
for end-users.  Our developers can run "git blame" to find this
commit as needed.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: gitster

contrib/completion/git-completion.bash

@@ -13,7 +13,8 @@
 #    *) git email aliases for git-send-email
 #    *) tree paths within 'ref:path/to/file' expressions
 #    *) file paths within current working directory and index
-#    *) common --long-options
+#    *) common --long-options but not single-letter options
+#    *) arguments to long and single-letter options
 #
 # To use these routines:
 #