feature #1083 Add sync a fork branch with the upstream repository

lib/Github/Api/Repo.php

@@ -535,6 +535,26 @@ public function branches($username, $repository, $branch = null, array $paramete
         return $this->get($url, $parameters);
     }
 
+
+    /**
+     * Sync a fork branch with the upstream repository.
+     *
+     * @link https://docs.github.com/en/rest/branches/branches#sync-a-fork-branch-with-the-upstream-repository
+     *
+     * @param $username
+     * @param $repository
+     * @param $branchName
+     *
+     * @return array|string
+     */
+    public function mergeUpstream($username, $repository, $branchName = null)
+    {
+        return $this->post(
+            '/repos/'.rawurlencode($username).'/'.rawurlencode($repository).'/merge-upstream',
+            ['branch' => $branchName ?? 'main']
+        );
+    }
+
     /**
      * Manage the protection of a repository branch.
      *