diff: discard hunk headers for patch-ids earlier

diff: discard hunk headers for patch-ids earlier

We do not include hunk header lines when computing patch-ids, since
the line numbers would create false negatives. Rather than detect and
skip them in our line callback, we can simply tell xdiff to avoid
generating them.

This is similar to the previous commit, but split out because it
actually requires modifying the matching line callback.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>