This bug was disovered by someone on IRC when he tried to

    $ git stash branch <branch> <stash>

while <branch> already existed. In that case the stash is dropped even
though it isn't applied on any branch, so the stash is effectively lost.
Signed-off-by: NTomas Carnecky <tom@dbservice.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>