range-diff: allow to diff files regardless of submodule config

If we have `submodule.diff = log' in the configuration file
or `--submodule=log' is given as argument, range-diff fails
to compare both diffs and we only get the following output:

    Submodule a 0000000...0000000 (new submodule)

Even if the repository doesn't have any submodule.

That's because the mode in diff_filespec is not correct and when
flushing the diff, down in builtin_diff() we will enter the condition:

	if (o->submodule_format == DIFF_SUBMODULE_LOG &&
	    (!one->mode || S_ISGITLINK(one->mode)) &&
	    (!two->mode || S_ISGITLINK(two->mode))) {
		show_submodule_summary(o, one->path ? one->path : two->path,
				&one->oid, &two->oid,
				two->dirty_submodule);
		return;

It turns out that S_ISGITLINK will return true (mode == 0160000 here).
Similar thing happens if submodule.diff is "diff".

Do like it's done in grep.c when calling fill_filespec() and force it to
be recognized as a file by adding S_IFREG to the mode.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

range-diff.c

@@ -354,7 +354,7 @@ static struct diff_filespec *get_filespec(const char *name, const char *p)
 {
 	struct diff_filespec *spec = alloc_filespec(name);
 
-	fill_filespec(spec, &null_oid, 0, 0644);
+	fill_filespec(spec, &null_oid, 0, 0100644);
 	spec->data = (char *)p;
 	spec->size = strlen(p);
 	spec->should_munmap = 0;

t/t3206-range-diff.sh

@@ -122,6 +122,35 @@ test_expect_success 'changed commit' '
 	test_cmp expected actual
 '
 
+test_expect_success 'changed commit with sm config' '
+	git range-diff --no-color --submodule=log topic...changed >actual &&
+	cat >expected <<-EOF &&
+	1:  4de457d = 1:  a4b3333 s/5/A/
+	2:  fccce22 = 2:  f51d370 s/4/A/
+	3:  147e64e ! 3:  0559556 s/11/B/
+	    @@ -10,7 +10,7 @@
+	      9
+	      10
+	     -11
+	    -+B
+	    ++BB
+	      12
+	      13
+	      14
+	4:  a63e992 ! 4:  d966c5c s/12/B/
+	    @@ -8,7 +8,7 @@
+	     @@
+	      9
+	      10
+	    - B
+	    + BB
+	     -12
+	     +B
+	      13
+	EOF
+	test_cmp expected actual
+'
+
 test_expect_success 'no commits on one side' '
 	git commit --amend -m "new message" &&
 	git range-diff master HEAD@{1} HEAD