Add too_large field to CommitDiffResponse
We used to apply this limitation on GitLab on https://gitlab.com/gitlab-org/gitlab-ce/blob/63daf37ad326bb4c4e74ebee8b8e2be7878b298f/lib/gitlab/git/diff.rb#L243-263.
Now that we're pruning diffs on Gitaly (gitaly!858 (merged)), we need a way to know what happened on GitLab.