Make pull request files backfill optional

[chingor13]

When fetching the list of commits via GraphQL, we return the list of files touched in the commit. The list of files is only available if there is an associated pull request. We are currently always backfilling the list of commits via the REST API.

In many circumstances the list of files touched is not needed and thus we are potentially making a lot of unnecessary API calls. We should make the helper that fetches the list of commits accept an optional parameter to fill in the list of files touched.