diff: make "too many files" rename warning optional

diff: make "too many files" rename warning optional

In many cases, the warning ends up as clutter, because the
diff is being done "behind the scenes" from the user (e.g.,
when generating a commit diffstat), and whether we show
renames or not is not particularly interesting to the user.

However, in the case of a merge (which is what motivated the
warning in the first place), it is a useful hint as to why a
merge with renames might have failed.

This patch makes the warning optional based on the code
calling into diffcore. We default to not showing the
warning, but turn it on for merges.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>