Rework the pre-commit hook to not actually edit code.

Change the pre-commit hook to work in the style of the one shipped with go
(/usr/local/Cellar/go//1.2/libexec/misc/git/pre-commit) which only tells the
user to fix their code.

Only perform checks on files that are touched by the change.

Change-Id: Ibb0ab322ec2f2480731d794846b48bd649a7ff26