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_commit_gogit.go

@@ -92,15 +92,14 @@ func (repo *Repository) getCommit(id ObjectID) (*Commit, error) {
 	}
 
 	commit := convertCommit(gogitCommit)
-	commit.repo = repo
 
 	tree, err := gogitCommit.Tree()
 	if err != nil {
 		return nil, err
 	}
 
-	commit.Tree.ID = ParseGogitHash(tree.Hash)
-	commit.Tree.resolvedGogitTreeObject = tree
+	commit.TreeID = ParseGogitHash(tree.Hash)
+	commit.Tree().resolvedGogitTreeObject = tree
 
 	return commit, nil
 }