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.

services/pull/merge_squash.go

@@ -25,7 +25,7 @@ func getAuthorSignatureSquash(ctx *mergeContext) (*git.Signature, error) {
 	// Try to get a signature from the same user in one of the commits, as the
 	// poster email might be private or commits might have a different signature
 	// than the primary email address of the poster.
-	gitRepo, err := git.OpenRepository(ctx.tmpBasePath)
+	gitRepo, err := git.OpenRepositoryLocal(ctx.tmpBasePath)
 	if err != nil {
 		log.Error("%-v Unable to open base repository: %v", ctx.pr, err)
 		return nil, err