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>

tests/integration/pull_merge_test.go

@@ -432,6 +432,6 @@ func TestConflictChecking(t *testing.T) {
 		// Check if status is correct.
 		assert.Equal(t, issues_model.PullRequestStatusConflict, conflictingPR.Status)
 		// Ensure that mergeable returns false
-		assert.False(t, conflictingPR.Mergeable())
+		assert.False(t, conflictingPR.Mergeable(db.DefaultContext))
 	})
 }