range-diff: suppress the diff headers

When showing the diff between corresponding patches of the two branch
versions, we have to make up a fake filename to run the diff machinery.

That filename does not carry any meaningful information, hence tbdiff
suppresses it. So we should, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/range-diff.c

@@ -33,6 +33,7 @@ int cmd_range_diff(int argc, const char **argv, const char *prefix)
 
 	diff_setup(&diffopt);
 	diffopt.output_format = DIFF_FORMAT_PATCH;
+	diffopt.flags.suppress_diff_headers = 1;
 	diffopt.output_prefix = output_prefix_cb;
 	strbuf_addstr(&four_spaces, "    ");
 	diffopt.output_prefix_data = &four_spaces;

diff.c

@@ -3395,13 +3395,16 @@ static void builtin_diff(const char *name_a,
 		memset(&xpp, 0, sizeof(xpp));
 		memset(&xecfg, 0, sizeof(xecfg));
 		memset(&ecbdata, 0, sizeof(ecbdata));
+		if (o->flags.suppress_diff_headers)
+			lbl[0] = NULL;
 		ecbdata.label_path = lbl;
 		ecbdata.color_diff = want_color(o->use_color);
 		ecbdata.ws_rule = whitespace_rule(name_b);
 		if (ecbdata.ws_rule & WS_BLANK_AT_EOF)
 			check_blank_at_eof(&mf1, &mf2, &ecbdata);
 		ecbdata.opt = o;
-		ecbdata.header = header.len ? &header : NULL;
+		if (header.len && !o->flags.suppress_diff_headers)
+			ecbdata.header = &header;
 		xpp.flags = o->xdl_opts;
 		xpp.anchors = o->anchors;
 		xpp.anchors_nr = o->anchors_nr;

diff.h

@@ -94,6 +94,7 @@ struct diff_flags {
 	unsigned funccontext:1;
 	unsigned default_follow_renames:1;
 	unsigned stat_with_summary:1;
+	unsigned suppress_diff_headers:1;
 };
 
 static inline void diff_flags_or(struct diff_flags *a,