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

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>