diff: drop complete_rewrite parameter from run_external_diff()

diff: drop complete_rewrite parameter from run_external_diff()

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