Add merge_requests_rebase_enabled to json entities for specs

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

@@ -97,7 +97,8 @@
     "commits_count": { "type": "integer" },
     "remove_source_branch": { "type": ["boolean", "null"] },
     "merge_ongoing": { "type": "boolean" },
-    "ff_only_enabled": { "type": ["boolean", false] }
+    "ff_only_enabled": { "type": ["boolean", false] },
+    "should_be_rebased": { "type": "boolean" }
   },
   "additionalProperties": false
 }

spec/lib/gitlab/import_export/safe_model_attributes.yml

@@ -413,6 +413,7 @@ Project:
 - ci_config_path
 - delete_error
 - merge_requests_ff_only_enabled
+- merge_requests_rebase_enabled
 Author:
 - name
 ProjectFeature: