add more debug info

models/pull.go

@@ -195,7 +195,7 @@ func (pr *PullRequest) Merge(doer *User, baseGitRepo *git.Repository) (err error
 	if _, stderr, err = process.ExecDir(-1, tmpBasePath,
 		fmt.Sprintf("PullRequest.Merge (git merge --no-commit): %s", tmpBasePath),
 		"git", "merge", "--no-ff", "--no-commit", "head_repo/"+pr.HeadBranch); err != nil {
-		return fmt.Errorf("git merge --no-ff --no-commit [%s]: %s", tmpBasePath, stderr)
+		return fmt.Errorf("git merge --no-ff --no-commit [%s]: %v - %s", tmpBasePath, err, stderr)
 	}
 
 	sig := doer.NewGitSig()