Our builtin_diff() wants to know whether break-detection found a
complete rewrite, because it changes how the diff is shown. However,
when calling out to an external diff, we don't pass this information
along (and doing so would require designing a new interface to the
user-provided program).
Let's drop the unused parameter to make this fact more clear.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>