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_create_test.go

@@ -4,6 +4,7 @@
 package integration
 
 import (
+	"fmt"
 	"net/http"
 	"net/http/httptest"
 	"net/url"
@@ -17,6 +18,23 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
+func createPullRequest(t *testing.T, session *TestSession, user, repo, baseBranch, headBranch, title string) *httptest.ResponseRecorder {
+	link := fmt.Sprintf("/%s/%s/compare/%s...%s", user, repo, baseBranch, headBranch)
+	req := NewRequest(t, "GET", link)
+	resp := session.MakeRequest(t, req, http.StatusOK)
+
+	// Submit the form for creating the pull
+	htmlDoc := NewHTMLParser(t, resp.Body)
+	link, exists := htmlDoc.doc.Find("form.ui.form").Attr("action")
+	assert.True(t, exists, "The template has changed")
+	req = NewRequestWithValues(t, "POST", link, map[string]string{
+		"_csrf": htmlDoc.GetCSRF(),
+		"title": title,
+	})
+	resp = session.MakeRequest(t, req, http.StatusOK)
+	return resp
+}
+
 func testPullCreate(t *testing.T, session *TestSession, user, repo, branch, title string) *httptest.ResponseRecorder {
 	req := NewRequest(t, "GET", path.Join(user, repo))
 	resp := session.MakeRequest(t, req, http.StatusOK)