When generating a range-diff, matching up commits between two version of
a patch series involves heuristics, thus may give unexpected results.
git-range-diff allows tweaking the heuristic via --creation-factor.
Follow suit by accepting --creation-factor in combination with
--range-diff when generating a range-diff for a cover-letter.
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>