"git format-patch --range-diff" by mistake passed the diff options
used to generate the primary output of the command to the
range-diff machinery, which caused the range-diff in the cover
letter to include fairly useless "--stat" output. This has been
corrected by forcing a non-customizable default formatting options
on the range-diff machinery when driven by format-patch.
* jc/format-patch-range-diff-fix:
format-patch: do not let its diff-options affect --range-diff