stash: learn to parse -m/--message like commit does

stash: learn to parse -m/--message like commit does

`git stash push -m foo` uses "foo" as the message for the stash. But
`git stash push -m"foo"` does not parse successfully.  Similarly
`git stash push --message="My stash message"` also fails.  The stash
documentation doesn't suggest this syntax should work, but gitcli
does and my fingers have learned this pattern long ago for `commit`.

Teach `git stash` to parse -mFoo and --message=Foo the same as `git
commit` would do.  Even though it's an internal function, add
similar support to create_stash() for consistency.

Signed-off-by: Phil Hord <phil.hord@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>