Fix entities/merge_request.json by adding ff_only_merge

spec/fixtures/api/schemas/entities/merge_request.json

@@ -96,7 +96,8 @@
     "remove_wip_path": { "type": "string" },
     "commits_count": { "type": "integer" },
     "remove_source_branch": { "type": ["boolean", "null"] },
-    "merge_ongoing": { "type": "boolean" }
+    "merge_ongoing": { "type": "boolean" },
+    "ff_only_enabled": { "type": ["boolean", false] }
   },
   "additionalProperties": false
 }