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/convert/pull.go

@@ -68,7 +68,7 @@ func ToAPIPullRequest(ctx context.Context, pr *issues_model.PullRequest, doer *u
 		PatchURL:  pr.Issue.PatchURL(),
 		HasMerged: pr.HasMerged,
 		MergeBase: pr.MergeBase,
-		Mergeable: pr.Mergeable(),
+		Mergeable: pr.Mergeable(ctx),
 		Deadline:  apiIssue.Deadline,
 		Created:   pr.Issue.CreatedUnix.AsTimePtr(),
 		Updated:   pr.Issue.UpdatedUnix.AsTimePtr(),