Remote update

This PR updates libgit2 to libgit2/libgit2@bcc622934a90307390f058c7ef4203f8bbcfe0d6.

There was an upstream change in that ```git_remote_create_inmemory``` has been replaced by ```git_remote_create_anonymous```. Some other bug fixes have also gone in, particularly related to cleanup of state files i.e. ```Git::Raw::Repository->cleanup_state()```.