Prior to this change, this was done through unicorn. In theory this
could time out. Workhorse has been sending these raw patches and diffs
for a long time and is stable in doing so.
Added bonus is the fact that `Commit#to_patch` can be removed.
`Commit#to_diff` too, which closes
https://gitlab.com/gitlab-org/gitaly/issues/324
Closes https://gitlab.com/gitlab-org/gitaly/issues/1196
