codeintel: Add commit updater utility

Created by: efritz

This PR adds utilities to update the commit graph for a repository. There is a general utility struct that can (blocking) update a repository in the query path (use in the code intel API will come in another PR), and a periodic background updater that will update all repositories marked as dirty (which is wired up to run in the worker in this PR).

This is part of a larger effort towards https://github.com/sourcegraph/sourcegraph/issues/12098.