Fix pull merge 500 error caused by git-fetch breaking behaviors (#8194)

Author: typeless

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, pr.HeadBranch).RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
+	if err := git.NewCommand("fetch", remoteRepoName, fmt.Sprintf("%s:refs/remotes/%s/%s", pr.HeadBranch, remoteRepoName, pr.HeadBranch)).RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
 		return fmt.Errorf("git fetch [%s -> %s]: %s", headRepoPath, tmpBasePath, errbuf.String())
 	}