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/mail_issue.go

@@ -82,7 +82,7 @@ func mailIssueCommentToParticipants(ctx *mailCommentContext, mentions []*user_mo
 
 	// =========== Repo watchers ===========
 	// Make repo watchers last, since it's likely the list with the most users
-	if !(ctx.Issue.IsPull && ctx.Issue.PullRequest.IsWorkInProgress() && ctx.ActionType != activities_model.ActionCreatePullRequest) {
+	if !(ctx.Issue.IsPull && ctx.Issue.PullRequest.IsWorkInProgress(ctx) && ctx.ActionType != activities_model.ActionCreatePullRequest) {
 		ids, err = repo_model.GetRepoWatchersIDs(ctx, ctx.Issue.RepoID)
 		if err != nil {
 			return fmt.Errorf("GetRepoWatchersIDs(%d): %w", ctx.Issue.RepoID, err)