Add test demonstrating the problem

When dropping changes to the submodule, we expect it to get rolled back to the
previous version; however, it is removed entirely instead.

pkg/integration/tests/test_list.go

@@ -119,6 +119,7 @@ var tests = []*components.IntegrationTest{
 	commit.CreateTag,
 	commit.DisableCopyCommitMessageBody,
 	commit.DiscardOldFileChanges,
+	commit.DiscardSubmoduleChanges,
 	commit.DoNotShowBranchMarkerForHeadCommit,
 	commit.FailHooksThenCommitNoHooks,
 	commit.FindBaseCommitForFixup,