Make prettify-changed script handle multi-lines

The git command outputs each filename on its own line, and passing that
to `prettier` fails.  Instead, using `xargs` seems to do the trick

scripts/prettify-changed.sh

@@ -8,7 +8,7 @@ get_changed() {
 prettify() {
     FILES=$1
     echo "Formatting files changed on this branch:"
-    ./node_modules/.bin/prettier --write "$FILES"
+    echo "$FILES" | xargs ./node_modules/.bin/prettier --write
     echo
 }