t4255: test am submodule with diff.submodule

git am will break when using diff.submodule=log; add some test cases
to illustrate this breakage as simply as possible.  There are
currently two ways this can fail:

* With errors ("unrecognized input"), if only change
* Silently (no submodule change), if other files change

Test for both conditions and ensure without diff.submodule this works.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Doug Kelly <dougk.ff7@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4255-am-submodule.sh

@@ -18,4 +18,76 @@ am_3way () {
 KNOWN_FAILURE_NOFF_MERGE_ATTEMPTS_TO_MERGE_REMOVED_SUBMODULE_FILES=1
 test_submodule_switch "am_3way"
 
+test_expect_success 'setup diff.submodule' '
+	test_commit one &&
+	INITIAL=$(git rev-parse HEAD) &&
+
+	git init submodule &&
+	(
+		cd submodule &&
+		test_commit two &&
+		git rev-parse HEAD >../initial-submodule
+	) &&
+	git submodule add ./submodule &&
+	git commit -m first &&
+
+	(
+		cd submodule &&
+		test_commit three &&
+		git rev-parse HEAD >../first-submodule
+	) &&
+	git add submodule &&
+	git commit -m second &&
+	SECOND=$(git rev-parse HEAD) &&
+
+	(
+		cd submodule &&
+		git mv two.t four.t &&
+		git commit -m "second submodule" &&
+		git rev-parse HEAD >../second-submodule
+	) &&
+	test_commit four &&
+	git add submodule &&
+	git commit --amend --no-edit &&
+	THIRD=$(git rev-parse HEAD) &&
+	git submodule update --init
+'
+
+run_test() {
+	START_COMMIT=$1 &&
+	EXPECT=$2 &&
+	# Abort any merges in progress: the previous
+	# test may have failed, and we should clean up.
+	test_might_fail git am --abort &&
+	git reset --hard $START_COMMIT &&
+	rm -f *.patch &&
+	git format-patch -1 &&
+	git reset --hard $START_COMMIT^ &&
+	git submodule update &&
+	git am *.patch &&
+	git submodule update &&
+	git -C submodule rev-parse HEAD >actual &&
+	test_cmp $EXPECT actual
+}
+
+test_expect_success 'diff.submodule unset' '
+	test_unconfig diff.submodule &&
+	run_test $SECOND first-submodule
+'
+
+test_expect_success 'diff.submodule unset with extra file' '
+	test_unconfig diff.submodule &&
+	run_test $THIRD second-submodule
+'
+
+test_expect_failure 'diff.submodule=log' '
+	test_config diff.submodule log &&
+	run_test $SECOND first-submodule
+'
+
+test_expect_failure 'diff.submodule=log with extra file' '
+	test_config diff.submodule log &&
+	run_test $THIRD second-submodule
+'
+
 test_done