Only update merge_base if not already merged (#10909)

* Only update merge_base if not already merged

Fix #10766

Signed-off-by: Andrew Thornton <art27@cantab.net>

* Prevent race in transfer pull request

* Update services/pull/pull.go

services/pull/check.go

@@ -47,7 +47,7 @@ func checkAndUpdateStatus(pr *models.PullRequest) {
 
 	// Make sure there is no waiting test to process before leaving the checking status.
 	if !pullRequestQueue.Exist(pr.ID) {
-		if err := pr.UpdateCols("merge_base", "status", "conflicted_files"); err != nil {
+		if err := pr.UpdateColsIfNotMerged("merge_base", "status", "conflicted_files"); err != nil {
 			log.Error("Update[%d]: %v", pr.ID, err)
 		}
 	}

services/pull/pull.go

@@ -115,7 +115,7 @@ func ChangeTargetBranch(pr *models.PullRequest, doer *models.User, targetBranch
 	if pr.Status == models.PullRequestStatusChecking {
 		pr.Status = models.PullRequestStatusMergeable
 	}
-	if err := pr.UpdateCols("status, conflicted_files, base_branch"); err != nil {
+	if err := pr.UpdateColsIfNotMerged("merge_base", "status", "conflicted_files", "base_branch"); err != nil {
 		return err
 	}