The files_count cache column was created with the assumption that the
number of files never changes throughout the life of a diff. However,
we have discovered the DeleteDiffFilesWorker, which removes them (and
sets the state of the diff to :without_files) in some limited cases.

This MR updates the cache columns (both commits_count and files_count)
so they will be re-recorded on every update. Since updates are rare,
the additional cost of this should be very low.