Stop leaking how the primary output of format-patch is customized to
the range-diff machinery and instead let the latter use its own
"reasonable default", in order to correct the breakage introduced by
a5170794

 ("Merge branch 'ab/range-diff-no-patch'", 2018-11-18) on
the 'master' front.  "git format-patch --range-diff..." without any
weird diff option started to include the "range-diff --stat" output,
which is rather useless right now, that made the whole thing
unusable and this is probably the least disruptive way to whip the
codebase into a shippable shape.

We may want to later make the range-diff driven by format-patch more
configurable, but that would have to wait until we have a good
design.

Signed-off-by: Junio C Hamano <gitster@pobox.com>