git kw expansion

I experimented a bit with the date kw expansion example in this page
that Rintze found …

… and what it does is to replace a keyword in the source file with
the expanded date (I believe of the last update for the file, but I
haven’t confirmed this part). Git itself, however, never sees that
date (in the file), and it’s only expanded on checkout. I think this
is what we want.

To wit, we could do something similar to what zotero does with the
styles in svn.

I still wish we could have a workflow that could properly validate
these missing-updated styles before commit.

Maybe we could have a script that inserts a dummy value for
validation, and then removes it?

Or, alternately, maybe I should create a schema variant just for this
use case that leaves the updated element optional?