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/issue/pull.go

@@ -68,7 +68,7 @@ func PullRequestCodeOwnersReview(ctx context.Context, pr *issues_model.PullReque
 
 	var data string
 	for _, file := range codeOwnerFiles {
-		if blob, err := commit.GetBlobByPath(file); err == nil {
+		if blob, err := commit.GetBlobByPath(ctx, repo, file); err == nil {
 			data, err = blob.GetBlobContent(setting.UI.MaxDisplayFileSize)
 			if err == nil {
 				break