Update commits for a repo during LSIF exists check

Created by: efritz

Please review by commit

This adds a missing bit of functionality that exists in the previous version. When a query comes in for a commit we don't know about, we ask gitserver for the ancestors of that commit then update the visibility of the dumps for that repo.

This allows us to determine the closest dump for commits we haven't seen before. This is a necessary step if you are viewing a commit newer than the latest dump to a branch.