Support Full File for Rapid Diffs diff_file endpoint

We should support the full param in the existing diff_file endpoint.

The full: true param means we need to have a diff with the expanded context (i.e. without any gaps).

It could be achieved by either:

1. Sending -U #{Gitlab::CurrentSettings.diff_max_patch_bytes} param to Git (through Gitaly API)
2. Grabbing the full blob by that path and merging the blob with the diff

Existing diff limits should also apply to this endpoint (i.e. we must return 413 response in case the result is too large).