gitaly_client: Fix intermixed tests for UserSquash and UserCommitFiles

Tests for the UserCommitFiles RPC are currently part of the tests for
UserSquash. This seems to be unintentional and doesn't make a whole lot
of sense.

Move out tests for UserCommitFiles into its own standalone context.

spec/lib/gitlab/gitaly_client/operation_service_spec.rb

@@ -436,6 +436,7 @@ RSpec.describe Gitlab::GitalyClient::OperationService do
           Gitlab::Git::Repository::GitError, "something failed")
       end
     end
+  end
 
   describe '#user_commit_files' do
     subject do
@@ -472,7 +473,6 @@ RSpec.describe Gitlab::GitalyClient::OperationService do
       it { expect(subject).to be_nil }
     end
   end
-  end
 
   describe '#user_commit_patches' do
     let(:patches_folder) { Rails.root.join('spec/fixtures/patchfiles') }