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_nogogit.go

@@ -88,7 +88,7 @@ func (repo *Repository) getCommitWithBatch(batch CatFileBatch, id ObjectID) (*Co
 		}
 		return repo.getCommitWithBatch(batch, tag.Object)
 	case "commit":
-		commit, err := CommitFromReader(repo, id, io.LimitReader(rd, info.Size))
+		commit, err := CommitFromReader(id, io.LimitReader(rd, info.Size))
 		if err != nil {
 			return nil, err
 		}