Adapter for gollum to use Rugged (libgit2) at the backend. See the
gollum wiki for more information on adapters. Currently gollum uses
grit as a backend by default, but since that is abandonware, the plan
is to make this adapter the default in the future.

Please note that this adapter is currently in beta. It passes the unit
tests for gollum and gollum-lib, but it needs more comprehensive
testing. Please report any issues that you encounter.