completion: implement and use --list-cmds=main,others

This is part of the effort to break down and provide commands by
category in machine-readable form. This could be helpful later on when
completion script switches to use --list-cmds for selecting
completable commands. It would be much easier for the user to choose
to complete _all_ commands instead of the default selection by passing
different values to --list-cmds in git-completino.bash.

While at there, replace "git help -a" in git-completion.bash with
--list-cmds since it's better suited for this task.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -839,7 +839,7 @@ __git_commands () {
 	then
 		printf "%s" "${GIT_TESTING_COMMAND_COMPLETION}"
 	else
-		git help -a|egrep '^  [a-zA-Z0-9]'
+		git --list-cmds=main,others
 	fi
 }