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.

routers/web/repo/setting/lfs.go

@@ -122,7 +122,7 @@ func LFSLocks(ctx *context.Context) {
 		return
 	}
 
-	gitRepo, err := git.OpenRepository(tmpBasePath)
+	gitRepo, err := git.OpenRepositoryLocal(tmpBasePath)
 	if err != nil {
 		log.Error("Unable to open temporary repository: %s (%v)", tmpBasePath, err)
 		ctx.ServerError("LFSLocks", fmt.Errorf("failed to open new temporary repository in: %s %w", tmpBasePath, err))