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_stats_test.go

@@ -13,7 +13,7 @@ import (
 
 func TestRepository_GetCodeActivityStats(t *testing.T) {
 	bareRepo1Path := filepath.Join(testReposDir, "repo1_bare")
-	bareRepo1, err := OpenRepository(bareRepo1Path)
+	bareRepo1, err := OpenRepositoryLocal(bareRepo1Path)
 	assert.NoError(t, err)
 	defer bareRepo1.Close()