Creating a new review request from committed change by selecting the first/root commit in a linked GitHub repository failed because the root commit has no parents. This raised a SCMError when trying to fetch the "compare two commits" API. 
The fix is to check if "parent_revision" is empty before trying to fetch "compare two commits". If "parent_revision" is empty, it fetches the root commit itself.

Manually created new review requests for a root commit with 1 changed file and for multiple changed files, and verified that "View Diff" listed the expected changes (for both a private and a public GitHub repository).
Ran unit tests.