Make the find-renames option follow the behaviour in git-diff, where it
resets the threshold when none is given. So, for instance,
"--find-renames=25 --find-renames" should result in the default
threshold (50%) instead of 25%.
Signed-off-by: Felipe Gonçalves Assis <felipegassis@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>