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

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>