completion: add and use --list-cmds=alias

completion: add and use --list-cmds=alias

By providing aliases via --list-cmds=, we could simplify command
collection code in the script. We only issue one git command. Before
this patch that is "git config", after it's "git --list-cmds=". In
"git help" completion case we actually reduce one "git" process (for
getting guides) but that call was added in this series so it does not
really count.

A couple of bash functions are removed because they are not needed
anymore. __git_compute_all_commands() and $__git_all_commands stay
because they are still needed for completing pager.* config and
without "alias" group, the result is still cacheable.

There is a slight (good) change in _git_help() with this patch: before
"git help <tab>" shows external commands (as in _not_ part of git) as
well as part of $__git_all_commands. We have finer control over
command listing now and can exclude that because we can't provide a
man page for external commands anyway.

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