Don't add ugly files in pre-commit; warn instead

As mentioned in #559.

hooks/pre-commit

@@ -1,10 +1,16 @@
-#!/bin/sh
+#!/bin/bash
 
 jsfiles=$(git diff --cached --name-only --diff-filter=ACM "*.js" "*.jsx" "*.ts" "*.tsx" | tr '\n' ' ')
 [ -z "$jsfiles" ] && exit 0
 
-# Prettify all staged .js files
-echo "$jsfiles" | xargs ./node_modules/.bin/prettier --write
+# Check if any of the files are ugly
+uglyFiles=()
+for jsfile in $jsfiles; do
+    diff "$jsfile" <($(npm bin)/prettier "$jsfile") >/dev/null || uglyFiles+=("$jsfile")
+done
 
-# Add back the modified/prettified files to staging
-echo "$jsfiles" | xargs git add
+if [ -n $uglyFiles ]; then
+    echo "Prettify your files first:"
+    echo '$(npm bin)/prettier --write' "${uglyFiles[@]}"
+    exit 1
+fi