range-diff: make diff option behavior (e.g. --stat) consistent

Make the behavior when diff options (e.g. "--stat") are passed
consistent with how "diff" behaves.

Before 73a834e9e2 ("range-diff: relieve callers of low-level
configuration burden", 2018-07-22) running range-diff with "--stat"
would produce stat output and the diff output, as opposed to how
"diff" behaves where once "--stat" is specified "--patch" also needs
to be provided to emit the patch output.

As noted in a previous change ("range-diff doc: add a section about
output stability", 2018-11-07) the "--stat" output with "range-diff"
is useless at the moment.

But we should behave consistently with "diff" in anticipation of such
output being useful in the future, because it would make for confusing
UI if "diff" and "range-diff" behaved differently when it came to how
they interpret diff options.

The new behavior is also consistent with the existing documentation
added in ba931edd28 ("range-diff: populate the man page",
2018-08-13). See "[...]also accepts the regular diff options[...]" in
git-range-diff(1).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

range-diff.c

@@ -433,7 +433,8 @@ int show_range_diff(const char *range1, const char *range2,
 		struct strbuf indent = STRBUF_INIT;
 
 		memcpy(&opts, diffopt, sizeof(opts));
-		opts.output_format |= DIFF_FORMAT_PATCH;
+		if (!opts.output_format)
+			opts.output_format = DIFF_FORMAT_PATCH;
 		opts.flags.suppress_diff_headers = 1;
 		opts.flags.dual_color_diffed_diffs = dual_color;
 		opts.output_prefix = output_prefix_cb;

t/t3206-range-diff.sh

@@ -134,43 +134,20 @@ test_expect_success 'changed commit with --no-patch diff option' '
 '
 
 test_expect_success 'changed commit with --stat diff option' '
-	four_spaces="    " &&
 	git range-diff --no-color --stat topic...changed >actual &&
 	cat >expected <<-EOF &&
 	1:  4de457d = 1:  a4b3333 s/5/A/
 	     a => b | 0
 	     1 file changed, 0 insertions(+), 0 deletions(-)
-	$four_spaces
 	2:  fccce22 = 2:  f51d370 s/4/A/
 	     a => b | 0
 	     1 file changed, 0 insertions(+), 0 deletions(-)
-	$four_spaces
 	3:  147e64e ! 3:  0559556 s/11/B/
 	     a => b | 0
 	     1 file changed, 0 insertions(+), 0 deletions(-)
-	$four_spaces
-	    @@ -10,7 +10,7 @@
-	      9
-	      10
-	     -11
-	    -+B
-	    ++BB
-	      12
-	      13
-	      14
 	4:  a63e992 ! 4:  d966c5c s/12/B/
 	     a => b | 0
 	     1 file changed, 0 insertions(+), 0 deletions(-)
-	$four_spaces
-	    @@ -8,7 +8,7 @@
-	     @@
-	      9
-	      10
-	    - B
-	    + BB
-	     -12
-	     +B
-	      13
 	EOF
 	test_cmp expected actual
 '