grid: note git revision when installing from a git checkout

Installing from a git checkout is a bad practice for production, but it's a
common thing during development. One problem I often face is that I have a
software release installed from a given revision and I want to make a small
change to the software, but not recompile everything. To achieve this, I need
to use the exact same git revision that was installed before, but most of the
time, I don't remember what revision I have been using last time I installed.

This change is about adding a buildout comment in the generated buildout.cfg
made to install the profile, with the revision that was used for installing, so
that we can re-install the same software again.