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.

modules/git/repo_test.go

@@ -15,7 +15,7 @@ import (
 
 func TestRepoIsEmpty(t *testing.T) {
 	emptyRepo2Path := filepath.Join(testReposDir, "repo2_empty")
-	repo, err := OpenRepository(emptyRepo2Path)
+	repo, err := OpenRepositoryLocal(emptyRepo2Path)
 	assert.NoError(t, err)
 	defer repo.Close()
 	isEmpty, err := repo.IsEmpty(t.Context())