Fix fetching commits by branch on GitHub.

When I fixed filtering of SVN commits by branch, several versions
ago, I broke filtering by branch on GitHub.

The New Review Request page was no longer passing the starting SHA,
which GitHub needed, resulting in all commits being returned. This call
has been fixed.

It's worth noting that the GitHub API allows for filtering by branch,
but you can't both use that and SHAs for pagination. The SHA is good
enough, if provided, so we'll continue to use that, falling back on a
branch if only a branch is provided. The code has been updated with
documentation to cover this.

Reviewed at https://reviews.reviewboard.org/r/6575/