Delete the PR branch if nothing to merge

src/Tools/Github/GithubMergeTool/GithubMergeTool.cs

@@ -121,6 +121,8 @@ git commit
             // 422 (Unprocessable Entity) indicates there were no commits to merge
             if (response.StatusCode == (HttpStatusCode)422)
             {
+                // Delete the pr branch if the PR was not created.
+                await _client.DeleteAsync($"repos/{repoOwner}/{repoName}/git/refs/heads/{prBranchName}");
                 return response;
             }