Fix codeowner detected diff base branch to mergebase (#29783) (#29807)

Fix #29763
Backport #29783 

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/mailer/notify.go

@@ -79,7 +79,7 @@ func (m *mailNotifier) IssueChangeTitle(ctx context.Context, doer *user_model.Us
 		log.Error("issue.LoadPullRequest: %v", err)
 		return
 	}
-	if issue.IsPull && issues_model.HasWorkInProgressPrefix(oldTitle) && !issue.PullRequest.IsWorkInProgress() {
+	if issue.IsPull && issues_model.HasWorkInProgressPrefix(oldTitle) && !issue.PullRequest.IsWorkInProgress(ctx) {
 		if err := MailParticipants(ctx, issue, doer, activities_model.ActionPullRequestReadyForReview, nil); err != nil {
 			log.Error("MailParticipants: %v", err)
 		}