Fix codeowner detected diff base branch to mergebase (#29783)

Fix #29763

This PR fixes 2 problems with CodeOwner in the pull request.
- Don't use the pull request base branch but merge-base as a diff base to
detect the code owner.
- CodeOwner detection in fork repositories will be disabled because
almost all the fork repositories will not change CODEOWNERS files but it
should not be used on fork repositories' pull requests.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/issue/issue.go

@@ -90,7 +90,7 @@ func ChangeTitle(ctx context.Context, issue *issues_model.Issue, doer *user_mode
 	}
 
 	if issue.IsPull && issues_model.HasWorkInProgressPrefix(oldTitle) && !issues_model.HasWorkInProgressPrefix(title) {
-		if err := issues_model.PullRequestCodeOwnersReview(ctx, issue, issue.PullRequest); err != nil {
+		if err := PullRequestCodeOwnersReview(ctx, issue, issue.PullRequest); err != nil {
 			return err
 		}
 	}