Reduce verbosity of `make format` output

Only log names of source files that were changed as a result of
auto-formatting.

package.json

@@ -147,7 +147,7 @@
     "deps": "check-dependencies",
     "lint": "eslint .",
     "checkformatting": "prettier --check 'src/**/*.js'",
-    "format": "prettier --write 'src/**/*.js'",
+    "format": "prettier --list-different --write 'src/**/*.js'",
     "test": "gulp test",
     "report-coverage": "codecov -f coverage/coverage-final.json",
     "version": "make clean build/manifest.json",