Our coding guidelines recommend following "The seven rules of a great
Git commit message" by Chris Beams:
https://chris.beams.io/posts/git-commit/
This change adds a Git commit hook that validates all Git commit
messages according to these rules (at least to the extent possible).
Currently, the hook simply prints a warning and a list of violations
in case of a non-conforming Git commit message. The commit is
otherwise accepted. This could be changed to entirely fail the commit,
or, via another hook, fail to push any code that is non-conformant.
The hook will be installed on a CMake run or when the hook source
changes.
