Add git hook for package comparison

When moving to commit, this hook will automatically fire and check the
current system's installed packages against those explicitly committed
to the repository. If they mismatch it will inform the user.

It will not prevent the commit, but simply add a comment at the top of
the commit messages to remind the user that something is unbalanced
between both. It would be recommended to either check the additional
package into source control, remove it from the current system, or
explicitly add it to ignored packages.

_bootstrap/.stow-local-ignore

@@ -1,4 +1,4 @@
 # TODO find a more generic way to express 'ignore any non-folder files'
 
 ^/install_packages.sh
-^/packages.csv
+^/packages.*.txt