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/issue.go

@@ -1910,7 +1910,7 @@ func ViewIssue(ctx *context.Context) {
 			if pull.HasMerged || issue.IsClosed || !ctx.IsSigned {
 				return false
 			}
-			if pull.CanAutoMerge() || pull.IsWorkInProgress() || pull.IsChecking() {
+			if pull.CanAutoMerge() || pull.IsWorkInProgress(ctx) || pull.IsChecking() {
 				return false
 			}
 			if (ctx.Doer.IsAdmin || ctx.Repo.IsAdmin()) && prConfig.AllowManualMerge {