repo-updater: Fix not-cloned count by comparing lowercase repo names

Created by: mrnugget

gitserver's list?cloned endpoint only returns lowercase repository names. In order for the computation of the not-cloned count in the status indicator to work properly, we need to lowercase the repository names we load from the database before comparing them to gitservers.

Discovered this by accident when cloning the complete github.com/sourcegraph organization and seeing the not-cloned count not going down after a certain point.

Test plan: go test