Merge branch 'jv/maint-no-ext-diff' into maint

"git diff --no-ext-diff" did not output anything for a typechange
filepair when GIT_EXTERNAL_DIFF is in effect.

* jv/maint-no-ext-diff:
  diff: test precedence of external diff drivers
  diff: correctly disable external_diff with --no-ext-diff

diff.c

@@ -2992,9 +2992,8 @@ static void run_diff_cmd(const char *pgm,
 	int complete_rewrite = (p->status == DIFF_STATUS_MODIFIED) && p->score;
 	int must_show_header = 0;
 
-	if (!DIFF_OPT_TST(o, ALLOW_EXTERNAL))
-		pgm = NULL;
-	else {
+
+	if (DIFF_OPT_TST(o, ALLOW_EXTERNAL)) {
 		struct userdiff_driver *drv = userdiff_find_by_path(attr_path);
 		if (drv && drv->external)
 			pgm = drv->external;
@@ -3074,6 +3073,9 @@ static void run_diff(struct diff_filepair *p, struct diff_options *o)
 	if (o->prefix_length)
 		strip_prefix(o->prefix_length, &name, &other);
 
+	if (!DIFF_OPT_TST(o, ALLOW_EXTERNAL))
+		pgm = NULL;
+
 	if (DIFF_PAIR_UNMERGED(p)) {
 		run_diff_cmd(pgm, name, NULL, attr_path,
 			     NULL, NULL, NULL, o, p);

t/t4020-diff-external.sh

@@ -48,7 +48,53 @@ test_expect_success 'GIT_EXTERNAL_DIFF environment and --no-ext-diff' '
 
 '
 
+test_expect_success SYMLINKS 'typechange diff' '
+	rm -f file &&
+	ln -s elif file &&
+	GIT_EXTERNAL_DIFF=echo git diff  | {
+		read path oldfile oldhex oldmode newfile newhex newmode &&
+		test "z$path" = zfile &&
+		test "z$oldmode" = z100644 &&
+		test "z$newhex" = "z$_z40" &&
+		test "z$newmode" = z120000 &&
+		oh=$(git rev-parse --verify HEAD:file) &&
+		test "z$oh" = "z$oldhex"
+	} &&
+	GIT_EXTERNAL_DIFF=echo git diff --no-ext-diff >actual &&
+	git diff >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'diff.external' '
+	git reset --hard &&
+	echo third >file &&
+	test_config diff.external echo &&
+	git diff | {
+		read path oldfile oldhex oldmode newfile newhex newmode &&
+		test "z$path" = zfile &&
+		test "z$oldmode" = z100644 &&
+		test "z$newhex" = "z$_z40" &&
+		test "z$newmode" = z100644 &&
+		oh=$(git rev-parse --verify HEAD:file) &&
+		test "z$oh" = "z$oldhex"
+	}
+'
+
+test_expect_success 'diff.external should apply only to diff' '
+	test_config diff.external echo &&
+	git log -p -1 HEAD |
+	grep "^diff --git a/file b/file"
+'
+
+test_expect_success 'diff.external and --no-ext-diff' '
+	test_config diff.external echo &&
+	git diff --no-ext-diff |
+	grep "^diff --git a/file b/file"
+'
+
 test_expect_success 'diff attribute' '
+	git reset --hard &&
+	echo third >file &&
 
 	git config diff.parrot.command echo &&
 
@@ -113,6 +159,19 @@ test_expect_success 'diff attribute and --no-ext-diff' '
 
 '
 
+test_expect_success 'GIT_EXTERNAL_DIFF trumps diff.external' '
+	>.gitattributes &&
+	test_config diff.external "echo ext-global" &&
+	GIT_EXTERNAL_DIFF="echo ext-env" git diff | grep ext-env
+'
+
+test_expect_success 'attributes trump GIT_EXTERNAL_DIFF and diff.external' '
+	test_config diff.foo.command "echo ext-attribute" &&
+	test_config diff.external "echo ext-global" &&
+	echo "file diff=foo" >.gitattributes &&
+	GIT_EXTERNAL_DIFF="echo ext-env" git diff | grep ext-attribute
+'
+
 test_expect_success 'no diff with -diff' '
 	echo >.gitattributes "file -diff" &&
 	git diff | grep Binary