rev-list: die when --test-bitmap detects a mismatch

You can use "git rev-list --test-bitmap HEAD" to check that bitmaps produce the same answer we'd get from a regular traversal. But if we detect an error, we only print "mismatch", and still exit with a successful error code. That makes the uses of --test-bitmap in the test suite (e.g., in t5310) mostly pointless: even if we saw an error, the tests wouldn't notice. Let's instead call die(), which will let these tests work as designed, and alert us if the bitmaps are bogus. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>