Fix panic on push at #7611 (#7615)

* Fix panic in #7611

Use pr.IssueID instead of pr.Issue.ID as Issue may not be loaded and is unnecessary

* Only fetch the head branch from the remote

models/branches.go

@@ -101,7 +101,7 @@ func (protectBranch *ProtectedBranch) HasEnoughApprovals(pr *PullRequest) bool {
 
 // GetGrantedApprovalsCount returns the number of granted approvals for pr. A granted approval must be authored by a user in an approval whitelist.
 func (protectBranch *ProtectedBranch) GetGrantedApprovalsCount(pr *PullRequest) int64 {
-	reviews, err := GetReviewersByPullID(pr.Issue.ID)
+	reviews, err := GetReviewersByPullID(pr.IssueID)
 	if err != nil {
 		log.Error("GetReviewersByPullID: %v", err)
 		return 0

modules/pull/merge.go

@@ -101,7 +101,7 @@ func Merge(pr *models.PullRequest, doer *models.User, baseGitRepo *git.Repositor
 	}
 
 	// Fetch head branch
-	if err := git.NewCommand("fetch", remoteRepoName).RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
+	if err := git.NewCommand("fetch", remoteRepoName, pr.HeadBranch).RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
 		return fmt.Errorf("git fetch [%s -> %s]: %s", headRepoPath, tmpBasePath, errbuf.String())
 	}