t7419: actually test the branch switching

The submodule repo the test set up had the 'topic' branch checked out,
meaning the repo's default branch (HEAD) is the 'topic' branch.

The following tests then pretended to switch between the default branch
and the 'topic' branch. This was papered over by continually adding
commits to the 'topic' branch and checking if the submodule gets updated
to this new commit.

Return the submodule repo to the 'main' branch after setup so we can
actually test the switching behavior.

Signed-off-by: Jan Alexander Steffens (heftig) <heftig@archlinux.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7419-submodule-set-branch.sh

@@ -11,6 +11,10 @@ as expected.
 
 TEST_PASSES_SANITIZE_LEAK=true
 TEST_NO_CREATE_REPO=1
+
+GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
+export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
+
 . ./test-lib.sh
 
 test_expect_success 'setup' '
@@ -27,7 +31,8 @@ test_expect_success 'submodule config cache setup' '
 		git checkout -b topic &&
 		echo b >a &&
 		git add . &&
-		git commit -mb
+		git commit -mb &&
+		git checkout main
 	) &&
 	mkdir super &&
 	(cd super &&
@@ -57,13 +62,12 @@ test_expect_success 'test submodule set-branch --branch' '
 '
 
 test_expect_success 'test submodule set-branch --default' '
-	test_commit -C submodule c &&
 	(cd super &&
 		git submodule set-branch --default submodule &&
 		! grep branch .gitmodules &&
 		git submodule update --remote &&
 		cat <<-\EOF >expect &&
-		c
+		a
 		EOF
 		git -C submodule show -s --pretty=%s >actual &&
 		test_cmp expect actual
@@ -71,7 +75,6 @@ test_expect_success 'test submodule set-branch --default' '
 '
 
 test_expect_success 'test submodule set-branch -b' '
-	test_commit -C submodule b &&
 	(cd super &&
 		git submodule set-branch -b topic submodule &&
 		grep "branch = topic" .gitmodules &&
@@ -85,13 +88,12 @@ test_expect_success 'test submodule set-branch -b' '
 '
 
 test_expect_success 'test submodule set-branch -d' '
-	test_commit -C submodule d &&
 	(cd super &&
 		git submodule set-branch -d submodule &&
 		! grep branch .gitmodules &&
 		git submodule update --remote &&
 		cat <<-\EOF >expect &&
-		d
+		a
 		EOF
 		git -C submodule show -s --pretty=%s >actual &&
 		test_cmp expect actual