Guard against data loss when snapshotting

As discussed in:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23508#note_123296899

Snapshotting doesn't use git, and will miss the required shared data.

app/workers/object_pool/create_worker.rb

@@ -37,8 +37,6 @@ module ObjectPool
       "object_pool:create:#{pool.id}"
     end
 
-    private
-
     def lease_timeout
       1.hour
     end

ee/app/services/geo/base_sync_service.rb

@@ -118,6 +118,10 @@ module Geo
     # will be enqueued by the log cursor, which should resolve any problems
     # it is possible to fix.
     def fetch_snapshot
+      # Snapshots will miss the data that are shared in object pools, and snapshotting should
+      # be avoided to guard against data loss.
+      return if project.pool_repository
+
       log_info("Attempting to fetch repository via snapshot")
 
       temp_repo.create_from_snapshot(