Add support for running Checkstyle on a given SHA-1.

Usage e.g.: ./checkstyle.py -s 3aca02ca91816a86febb5ed6e380ec2122adea47

Additionally this CL adds a pre-push script that can be used a git
hook to run Checkstyle on 'repo upload'

Bug: 25852971
Change-Id: Ia00d48df80b2b024de0899d9590868016c5b7bf0
4 files changed