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.

models/migrations/v1_12/v136.go

@@ -83,7 +83,7 @@ func AddCommitDivergenceToPulls(x db.EngineMigration) error {
 				log.Error("Missing base repo with id %d for PR ID %d", pr.BaseRepoID, pr.ID)
 				continue
 			}
-			repoStore := repo_model.StorageRepo(repo_model.RelativePath(baseRepo.OwnerName, baseRepo.Name))
+			repoStore := repo_model.CodeRepoByName(baseRepo.OwnerName, baseRepo.Name)
 			gitRefName := fmt.Sprintf("refs/pull/%d/head", pr.Index)
 			divergence, err := gitrepo.GetDivergingCommits(graceful.GetManager().HammerContext(), repoStore, pr.BaseBranch, gitRefName)
 			if err != nil {