stash: use stash_push for no verb form

stash: use stash_push for no verb form

Now that we have stash_push, which accepts pathspec arguments, use
it instead of stash_save in git stash without any additional verbs.

Previously we allowed git stash -- -message, which is no longer allowed
after this patch.  Messages starting with a hyphen was allowed since
3c2eb80f, ("stash: simplify defaulting to "save" and reject unknown
options").  However it was never the intent to allow that, but rather it
was allowed accidentally.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>