Manually set total_count when paginating commits

`Kaminari.paginate_array` takes some options, most relevant of which is
a `total_count` parameter. Using the `commit_count` for `total_count`
lets us correctly treat the return of `Repository#commits` as a subset
of all the commits we may wish to list.

Addition of a new `Repository#commit_count_for_ref` method was
necessarry to allow the user to start from an arbitrary ref.

Ref #1381