diff: convert --check to use a hunk callback

diff: convert --check to use a hunk callback

The "diff --check" code needs to know the line number on which each hunk
starts in order to generate its output. We get that now by parsing the
hunk header line generated by xdiff, but it's much simpler to just pass
it directly using a hunk callback.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>