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>

routers/web/repo/pull.go

@@ -706,7 +706,7 @@ func PrepareViewPullInfo(ctx *context.Context, issue *issues_model.Issue) *git.C
 		ctx.Data["IsNothingToCompare"] = true
 	}
 
-	if pull.IsWorkInProgress() {
+	if pull.IsWorkInProgress(ctx) {
 		ctx.Data["IsPullWorkInProgress"] = true
 		ctx.Data["WorkInProgressPrefix"] = pull.GetWorkInProgressPrefix(ctx)
 	}