Cloning the full source tree rather than just a snapshot at the desired
version does increase the size of the checkout, but not as much as
originally feared. It goes from 25 GB -> 31 GB, which is manageable.
On the other hand, this allows some automated tools to figure out the
commit hashes from the repository history, which prevents the frontend
from complaining.

I plan to clean up the buildout file in the future, but I just wanted
to commit a known-working version.