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/repository/gitgraph/graph_test.go

@@ -16,7 +16,7 @@ import (
 )
 
 func BenchmarkGetCommitGraph(b *testing.B) {
-	currentRepo, err := git.OpenRepository(".")
+	currentRepo, err := git.OpenRepositoryLocal(".")
 	if err != nil || currentRepo == nil {
 		b.Error("Could not open repository")
 	}