Fix Pull Merge when tag with same name as base branch exist (#13882)

fix dst refspec error in 'Push back to upstream' when base branch have
same name with a tag.

fix #13851
Signed-off-by: a1012112796 <1012112796@qq.com>

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: zeripath <art27@cantab.net>

services/pull/merge.go

@@ -411,7 +411,7 @@ func rawMerge(pr *models.PullRequest, doer *models.User, mergeStyle models.Merge
 	)
 
 	// Push back to upstream.
-	if err := git.NewCommand("push", "origin", baseBranch+":"+pr.BaseBranch).RunInDirTimeoutEnvPipeline(env, -1, tmpBasePath, &outbuf, &errbuf); err != nil {
+	if err := git.NewCommand("push", "origin", baseBranch+":refs/heads/"+pr.BaseBranch).RunInDirTimeoutEnvPipeline(env, -1, tmpBasePath, &outbuf, &errbuf); err != nil {
 		if strings.Contains(errbuf.String(), "non-fast-forward") {
 			return "", &git.ErrPushOutOfDate{
 				StdOut: outbuf.String(),