refactor: git repo and relative path handling (#38522)

1. simplify "StorageRepo" code
2. simplify git.OpenRepository code
3. by the way, clean up some unused files in git test fixtures.

services/convert/action_test.go

@@ -62,7 +62,7 @@ func TestGetActionWorkflow_FallbackRef(t *testing.T) {
 
 	repoDir := buildWorkflowTestRepo(t)
 
-	gitRepo, err := git.OpenRepository(repoDir)
+	gitRepo, err := git.OpenRepositoryLocal(repoDir)
 	require.NoError(t, err)
 	defer gitRepo.Close()