The logic that genates the list of commits to submit for a review request with commit history was previously attempting to naively convert the the revisions (which were text) to bytes. This is fine on Python 2, where `bytes(s)` implies `s.encode('ascii')` but this is no longer the case on Python 3. Instead, `str.encode()` must be called and it must be provided with a codec. This method works on both Python 2 and Python 3. Testing done: Confirmed the following hold on Python verisons 2.7, 3.5, 3.6, and 3.7: - With this entire patch stack (/r/10685, /r/10695, and /r/10697), `rbt post` is able to create review requests with commit history.