format-patch: do not let its diff-options affect --range-diff

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>

builtin/log.c

@@ -1096,7 +1096,7 @@ static void make_cover_letter(struct rev_info *rev, int use_stdout,
 	if (rev->rdiff1) {
 		fprintf_ln(rev->diffopt.file, "%s", rev->rdiff_title);
 		show_range_diff(rev->rdiff1, rev->rdiff2,
-				rev->creation_factor, 1, &rev->diffopt);
+				rev->creation_factor, 1, NULL);
 	}
 }