range-diff: indent the diffs just like tbdiff

The main information in the `range-diff` view comes from the list of
matching and non-matching commits, the diffs are additional information.
Indenting them helps with the reading flow.

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

builtin/range-diff.c

@@ -11,6 +11,11 @@ N_("git range-diff [<options>] <base> <old-tip> <new-tip>"),
 NULL
 };
 
+static struct strbuf *output_prefix_cb(struct diff_options *opt, void *data)
+{
+	return data;
+}
+
 int cmd_range_diff(int argc, const char **argv, const char *prefix)
 {
 	int creation_factor = 60;
@@ -21,12 +26,16 @@ int cmd_range_diff(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 	int i, j, res = 0;
+	struct strbuf four_spaces = STRBUF_INIT;
 	struct strbuf range1 = STRBUF_INIT, range2 = STRBUF_INIT;
 
 	git_config(git_diff_ui_config, NULL);
 
 	diff_setup(&diffopt);
 	diffopt.output_format = DIFF_FORMAT_PATCH;
+	diffopt.output_prefix = output_prefix_cb;
+	strbuf_addstr(&four_spaces, "    ");
+	diffopt.output_prefix_data = &four_spaces;
 
 	argc = parse_options(argc, argv, NULL, options,
 			     builtin_range_diff_usage, PARSE_OPT_KEEP_UNKNOWN |
@@ -90,6 +99,7 @@ int cmd_range_diff(int argc, const char **argv, const char *prefix)
 
 	strbuf_release(&range1);
 	strbuf_release(&range2);
+	strbuf_release(&four_spaces);
 
 	return res;
 }