dir.c::match_pathname(): adjust patternlen when shifting pattern

If we receive a pattern that starts with "/", we shift it forward to avoid looking at the "/" part. Since the prefix and patternlen parameters are counts of what is in the pattern, we must decrement them as we increment the pointer. We remembered to handle prefix, but not patternlen. This didn't cause any bugs, though, because the patternlen parameter is not actually used. Since it will be used in future patches, let's correct this oversight. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>