"gn format" changed to remove the --in-place argument. This updates the
instructions for formatting and fixes existing usage.

git cl format --diff never worked, it just printed the full formatted output
unconditionally. This change improves things slightly by indicating there's a
diff for a file. A proper diff can be hooked up in a later stage if somebody
so desires.

Review-Url: https://codereview.chromium.org/2220123002