update git-fixtures to fix Node 14 tests

Author: mansona

package-lock.json

@@ -65,7 +65,7 @@
     "eslint-plugin-node": "^11.1.0",
     "eslint-plugin-prefer-let": "^3.0.1",
     "git-diff-apply": "^3.0.0",
-    "git-fixtures": "^4.0.0",
+    "git-fixtures": "^6.0.0",
     "mocha": "^10.0.0",
     "mocha-helpers": "^6.2.1",
     "renovate-config-standard": "2.1.2",