Propellor.Property.Git.cloned clones a git repo to a location, great! Is there an easy way to keep it updated afterwards, though? I guess having a property that enforce up-to-dateness wouldn't be so practical since you have to hit the remote to check, perhaps a cron job to run some equivalent of git fetch && git reset --hard && git clean -dfx. (Just thinking out loud here, and wondering what everyone else is doing)
You could certianly write a property that did a git pull every time. It would do some unnessesary work and still not keep the checkout always immediately up-to-date.
Better to have some kind of hook that causes the checkout to update when changes are pushed into its parent repository. How such a hook works depends on where the parent repository is hosted. Propellor properties could be written to set up whatever's needed for such a hook on the system where the repository is checked out. If you're hosting your own git server as well, propellor properties could set up the post-update hook in the origin repo too.