Add initial version of git commit message preparation hook

Start with a slightly modified version of the SVN commit template, to
allow developers to experiment. This will be updated in the future as
our process and techniques evolve.

This can be installed by copying or symlinking into the .git/hooks/
directory.

Feedback from: cem, jhb
Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D27633