stash: strip "refs/heads/" with skip_prefix

stash: strip "refs/heads/" with skip_prefix

When generating a message for a stash, "git stash" only records the
part of the branch name to the right of the last "/". e.g. if HEAD is at
"foo/bar/baz", "git stash" generates a message prefixed with "WIP on
baz:" instead of "WIP on foo/bar/baz:".

Fix this by using skip_prefix() to skip "refs/heads/" instead of looking
for the last instance of "/".

Reported-by: Kraymer <kraymer@gmail.com>
Reported-by: Daniel Hahler <git@thequod.de>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>