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/pull/check.go

@@ -91,7 +91,7 @@ func CheckPullMergable(stdCtx context.Context, doer *user_model.User, perm *acce
 			return nil
 		}
 
-		if pr.IsWorkInProgress() {
+		if pr.IsWorkInProgress(ctx) {
 			return ErrIsWorkInProgress
 		}