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/pull/pull.go

@@ -136,7 +136,7 @@ func NewPullRequest(ctx context.Context, repo *repo_model.Repository, issue *iss
 		}
 
 		if !pr.IsWorkInProgress(ctx) {
-			if err := issues_model.PullRequestCodeOwnersReview(ctx, issue, pr); err != nil {
+			if err := issue_service.PullRequestCodeOwnersReview(ctx, issue, pr); err != nil {
 				return err
 			}
 		}