Move batching to oids array from AR query

Since oids can be an unbounded array, extremely large sets of oids can
cause query parsing problems in PG, leading to timeoutes, clogging logs,
etc.

app/services/projects/lfs_pointers/lfs_link_service.rb

@@ -23,7 +23,9 @@ module Projects
         all_existing_objects = []
         iterations = 0
 
-        LfsObject.where(oid: oids).each_batch(of: BATCH_SIZE) do |existent_lfs_objects|
+        oids.each_slice(BATCH_SIZE) do |oids_batch|
+          existent_lfs_objects = LfsObject.where(oid: oids_batch)
+
           next unless existent_lfs_objects.any?
 
           iterations += 1

spec/services/projects/lfs_pointers/lfs_link_service_spec.rb

@@ -48,5 +48,15 @@ describe Projects::LfsPointers::LfsLinkService do
       expect(project.all_lfs_objects.count).to eq 9
       expect(linked.size).to eq 7
     end
+
+    it 'only queries for the batch that will be processed', :aggregate_failures do
+      stub_const("#{described_class}::BATCH_SIZE", 1)
+      oids = %w(one two)
+
+      expect(LfsObject).to receive(:where).with(oid: %w(one)).once.and_call_original
+      expect(LfsObject).to receive(:where).with(oid: %w(two)).once.and_call_original
+
+      subject.execute(oids)
+    end
   end
 end