xdi_diff: trim common trailing lines

xdi_diff: trim common trailing lines

This implements earlier Linus's optimization to trim common lines at the
end before passing them down to low level xdiff interface for all of our
xdiff users.

We could later enhance this to also trim common leading lines, but that
would need tweaking the output function to add the number of lines
trimmed at the beginning to line numbers that appear in the hunk
headers.

Signed-off-by: Junio C Hamano <gitster@pobox.com>