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.

tests/integration/git_general_test.go

@@ -811,7 +811,7 @@ func doCreateAgitFlowPull(dstPath string, ctx *APITestContext, headBranch string
 			return
 		}
 
-		gitRepo, err := git.OpenRepository(dstPath)
+		gitRepo, err := git.OpenRepositoryLocal(dstPath)
 		require.NoError(t, err)
 
 		defer gitRepo.Close()