git-completion.bash: use __gitcomp_builtin() in _git_stash()

git-completion.bash: use __gitcomp_builtin() in _git_stash()

The completion for 'git stash' has not changed in a major way since it
was converted from shell script to builtin. Now that it's a builtin, we
can take advantage of the groundwork laid out by parse-options and use
the generated options.

Rewrite _git_stash() to take use __gitcomp_builtin() to generate
completions for subcommands.

The main `git stash` command does not take any arguments directly. If no
subcommand is given, it automatically defaults to `git stash push`. This
means that we can simplify the logic for when no subcommands have been
given yet. We only have to offer subcommand completions when we're
completing a non-option after "stash".

One area that this patch could improve upon is that the `git stash list`
command accepts log-options. It would be nice if the completion for this
were unified with that of _git_log() and _git_show() which would allow
completions to be provided for options such as `--pretty` but that is
outside the scope of this patch.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>