Add the prepare-commit-msg hook to the repository

This is a merged and modified version from src and ports repositories.

The easiest way to install is run the following command in the repository:

git config --add core.hooksPath .hooks

or copy it to the hooks directory, but it will not get automatically updated:

cp .hooks/prepare-commit-msg .git/hooks/

Approved by: carlavilla (doceng)
Differential Revision: https://reviews.freebsd.org/D36203