#269 - MkRepoCommits should be able to get commit.

I created the MkRepoCommit with some puzzles as stated on this comment https://github.com/jcabi/jcabi-github/issues/269#issuecomment-35790490. I added the login param to the MkRepoCommits since it was needed by the MkRepo constructor (which I need to pass to the MkRepoCommit).