Fixed the clonen of new repositories.

[gou4shi1]

Fix https://github.com/maralla/pack/issues/50 https://github.com/raphaelahrens/pack/commit/b19d2cba2bcdc534988a7759364c608b4173e3bd#commitcomment-66279418

When a plugin is under a different branch then the configured default branch, the old clone failed because it could not knwo which branch to fetch. I switched the cloning to the default Repository::clone_recursive, which handles different default branches better.