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>

Documentation/git-format-patch.txt

@@ -250,6 +250,11 @@ feeding the result to `git send-email`.
 	feature/v2`), or a revision range if the two versions of the series are
 	disjoint (for example `git format-patch --cover-letter
 	--range-diff=feature/v1~3..feature/v1 -3 feature/v2`).
++
+Note that diff options passed to the command affect how the primary
+product of `format-patch` is generated, and they are not passed to
+the underlying `range-diff` machinery used to generate the cover-letter
+material (this may change in the future).
 --creation-factor=<percent>::
 	Used with `--range-diff`, tweak the heuristic which matches up commits

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);
 	}
 }

log-tree.c

@@ -762,7 +762,7 @@ void show_log(struct rev_info *opt)
 		next_commentary_block(opt, NULL);
 		fprintf_ln(opt->diffopt.file, "%s", opt->rdiff_title);
 		show_range_diff(opt->rdiff1, opt->rdiff2,
-				opt->creation_factor, 1, &opt->diffopt);
+				opt->creation_factor, 1, NULL);
 		memcpy(&diff_queued_diff, &dq, sizeof(diff_queued_diff));
 	}

range-diff.c

@@ -460,7 +460,11 @@ int show_range_diff(const char *range1, const char *range2,
 		struct diff_options opts;
 		struct strbuf indent = STRBUF_INIT;
-		memcpy(&opts, diffopt, sizeof(opts));
+		if (diffopt)
+			memcpy(&opts, diffopt, sizeof(opts));
+		else
+			diff_setup(&opts);
+
 		if (!opts.output_format)
 			opts.output_format = DIFF_FORMAT_PATCH;
 		opts.flags.suppress_diff_headers = 1;

range-diff.h

@@ -5,6 +5,11 @@
 #define RANGE_DIFF_CREATION_FACTOR_DEFAULT 60
+/*
+ * Compare series of commmits in RANGE1 and RANGE2, and emit to the
+ * standard output.  NULL can be passed to DIFFOPT to use the built-in
+ * default.
+ */
 int show_range_diff(const char *range1, const char *range2,
 		    int creation_factor, int dual_color,
 		    struct diff_options *diffopt);