In https://gitlab.com/gitlab-org/gitaly/issues/1686 and
https://gitlab.com/gitlab-com/gl-infra/scalability/issues/20, we saw a
lot of I/O consumed by Git garbage collection running on pull mirrors.

It looks like the problem happened because
`Projects::AfterImportService` always ran after completion of a project
import sync, which caused `HousekeepingService` to run. Since the
`pushes_since_gc` counter never changed, `HousekeepingService` would
always run GC again.

This commit increments the `pushes_since_gc` counter so that GCs are
only run every 200 syncs. Ideally we'd only run `HousekeepingService`
only if the repository actually changed, but right now we don't have
an easy way of detecting that.

Note that this problem does NOT happen if the mirror is a imported
directly via a URL. It only happens if the `import_type` is
`Gitlab::ImportSources.importer_names`.

Closes https://gitlab.com/gitlab-org/gitaly/issues/1686