add xdi_diff_hunks() for callers that only need hunk lengths

add xdi_diff_hunks() for callers that only need hunk lengths

Based on a patch by Brian Downing, this uses the xdiff emit_func feature
to implement xdi_diff_hunks().  It's a function that calls a callback for
each hunk of a diff, passing its lengths.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>