repo-updater: Fix syncRepo logic

Created by: ryanslade

syncRepo syncs a single repo that has been sourced from a single code host. It is usually triggered by lazy syncing. Since it is only sourced from a single code host its Sources map only contained mapping for that one source. Our diff algorithm thus triggered the removal of the repo from all other sources in the external_service_repos table.

Instead, we now assume that the current stored mapping is correct for all OTHER sources.

Closes: https://github.com/sourcegraph/sourcegraph/issues/15820