Rename `BatchCommitLoader` to `BatchLfsOidLoader`

Since we're not actually  loading commits in that loader, but we're
loading blobs with LFS oids in batch and returning only the oids.

app/graphql/types/tree/blob_type.rb

@@ -10,7 +10,7 @@ module Types
 
       field :web_url, GraphQL::STRING_TYPE, null: true
       field :lfs_oid, GraphQL::STRING_TYPE, null: true, resolve: -> (blob, args, ctx) do
-        Gitlab::Graphql::Loaders::BatchCommitLoader.new(blob.repository, blob.id).find
+        Gitlab::Graphql::Loaders::BatchLfsOidLoader.new(blob.repository, blob.id).find
       end
     end
   end

lib/gitlab/graphql/loaders/batch_lfs_oid_loader.rb

@@ -3,7 +3,7 @@
 module Gitlab
   module Graphql
     module Loaders
-      class BatchCommitLoader
+      class BatchLfsOidLoader
         def initialize(repository, blob_id)
           @repository, @blob_id = repository, blob_id
         end

spec/lib/gitlab/graphql/loaders/batch_lfs_oid_loader_spec.rb

@@ -1,6 +1,6 @@
 require 'spec_helper'
 
-describe Gitlab::Graphql::Loaders::BatchCommitLoader do
+describe Gitlab::Graphql::Loaders::BatchLfsOidLoader do
   include GraphqlHelpers
 
   set(:project) { create(:project, :repository) }