Fixes #311.

Change-Id: I334b9e22e748d96a62b0c7f59690c1a998a3c802

this method actually returns RepositoryCommits (go-github's term for
this, not GitHub's) rather than Commits.  They are very, very close in
structure (which is likely why this has gone unnoticed for so long), one
of the primary differences being that RepositoryCommits exposes the
GitHub identity of the commit author and committer if possible.