Add MaxPatchBytes to CommitDiffRequest RPC
With gitaly!858 (merged) we've hardcoded the 'single diff file' byte limitation, which was previously handled on GitLab side in a SIZE_LIMIT
constant.
Turns out we need to be able to send this value from GitLab in order to fix https://gitlab.com/gitlab-org/gitlab-ce/issues/50635 and work on https://gitlab.com/gitlab-org/gitlab-ce/issues/48027.