refactor: decouple git.Repository(ctx) from git.Commit & git.Tree (#38464)

1. Storing "ctx" in a long-living object is wrong
2. Make the commit & tree cacheable (for the future performance
optimization)
3. Also fix some bad designs like `// FIXME: bad design, this field can
be nil if the commit is from "last commit cache"`

ref:
* #33893

modules/git/repo_index.go

@@ -149,5 +149,5 @@ func (repo *Repository) WriteTree() (*Tree, error) {
 	if err != nil {
 		return nil, err
 	}
-	return NewTree(repo, id), nil
+	return newTree(id), nil
 }