filter-branch: drop $_x40 glob

When checking whether a commit was rewritten to a single object id, we use a glob that insists on a 40-hex result. This works for sha1, but fails t7003 when run with GIT_TEST_DEFAULT_HASH=sha256. Since the previous commit simplified the case statement here, we only have two arms: an empty string or a single object id. We can just loosen our glob to match anything, and still distinguish those cases (we lose the ability to notice bogus input, but that's not a problem; we are the one who wrote the map in the first place, and anyway update-ref will complain loudly if the input isn't a valid hash). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>