diffcore-break: use a goto instead of a redundant if statement

diffcore-break: use a goto instead of a redundant if statement

The condition "if (q->nr <= j)" checks whether the loop exited normally
or via a break statement. Avoid this check by replacing the jump out of
the inner loop with a jump to the end of the outer loop, which makes it
obvious that diff_q is not executed when the peer survives.

Signed-off-by: Alex Henrie <alexhenrie24@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>