app: use git restore to re-create .gitkeep file

This uses the more reliable way to restore the file in git after a
build, preventing differences in line endings to cause the file to be
seen as dirty in some situations.

Author: guggero

app/package.json

@@ -8,7 +8,7 @@
     "start": "BROWSER=none react-scripts start",
     "develop": "REACT_APP_USE_SAMPLE_DATA=true yarn start",
     "build": "react-scripts build",
-    "postbuild": "echo '# Keep directory in git.' > build/.gitkeep",
+    "postbuild": "git restore build/.gitkeep",
     "test": "react-scripts test --env=jest-environment-jsdom --transformIgnorePatterns \"node_modules/(?!d3)/\"",
     "test:ci": "cross-env CI=true yarn test --coverage",
     "eject": "react-scripts eject",