Fix creating a new review request from the first committed change

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.

Testing Done:
- 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.

Bugs closed: 3207

Reviewed at https://reviews.reviewboard.org/r/5321/