run-phpcs-mw.sh: wrapper around phpcs for CI needs

This wrapper let us run phpcs on all the files in the workspace or only
on files modified in HEAD (with git-changed-in-head). Simply pass 'head'
as an argument.

I have also added the ability to skip warnings entirely with
'nowarnings'.

bug: 48420
Change-Id: Ib3040b9014ec01b01e0e9cd74be411aab41d003e