LSIF: Stop inserting rows into lsif_commits we don't use
Created by: efritz
We only need lsif_commits entries in order to find dumps. If there are no dumps for a given repository, we shouldn't populate commits.
There are currently > 7mil rows in lsif_commits in dot-com, but we only have a handful of dumps.
Note for @sourcegraph/distribution : it is completely safe to truncate the lsif_commits table in a customer instance. It will rebuild itself on demand.