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.

modules/git/git_test.go

@@ -10,6 +10,8 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
+const testReposDir = "tests/repos/"
+
 func TestMain(m *testing.M) {
 	RunGitTests(m)
 }