stash: allow pathspecs in the no verb form

stash: allow pathspecs in the no verb form

Now that stash_push is used in the no verb form of stash, allow
specifying the command line for this form as well.  Always use -- to
disambiguate pathspecs from other non-option arguments.

Also make git stash -p an alias for git stash push -p.  This allows
users to use git stash -p <pathspec>.

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