When we save merge request diffs to the database, we need to expand the diff
before doing so. That's so that we can expand diffs (within the normal limits)
without hitting the repository, but just by going to the database.

This is done implicitly - diffs are expanded unless we say otherwise. However,
we have another option we can pass, that lets us enforce diff size limits, that
defaults to true.

Prior to this commit:

- The Rugged code path defaulted to setting `expanded: true` and
  `enforce_limits: true`.
- The Gitaly code path defaulted to setting `expanded: false` and
  `enforce_limits: true`.

This was introduced by eb36fa17, which
implemented the initial feature. Since then, if the `gitaly_diff_between`
feature flag was enabled, MRs would have diffs that could not be expanded in
some cases, with no fix other than to disable the feature flag and force push to
the MR to refresh the diff in the database.