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

services/gitdiff/git_diff_tree.go

@@ -100,7 +100,7 @@ func validateGitDiffTreeArguments(gitRepo *git.Repository, useMergeBase bool, ba
 			return false, objectFormat.EmptyTree().String(), headCommitID, nil
 		}
 
-		baseCommit, err := headCommit.Parent(0)
+		baseCommit, err := headCommit.Parent(gitRepo, 0)
 		if err != nil {
 			return false, "", "", fmt.Errorf("baseSha is '', attempted to use parent of commit %s, got error: %v", headCommit.ID.String(), err)
 		}