diff.c: respect diff.renames config option

diff.c: respect diff.renames config option

diff.renames is mentioned several times in the documentation,
but to my surprise it didn't do anything before this patch.

Also add the --no-renames option to override this from the
command-line.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>