detached-stash: simplify git stash branch

detached-stash: simplify git stash branch

This patch teaches git stash branch to tolerate stash-like arguments.

In particular, a stash is only required if an argument isn't specified
and the stash is only dropped if a stash entry reference was
specified or implied.

The implementation has been simplified by taking advantage of
assert_stash_like() and the variables established by
parse_flags_and_rev().

Signed-off-by: Jon Seymour <jon.seymour@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>