update libgit2 calls to match last Libgit2 API
update libgit2 calls to match last Libgit2 API

* git_threads_init was renamed to git_libgit2_init
* git_threads_shutdown was renamed to git_libgit2_shutdown

falcon-org/Falcon,falcon-org/Falcon,falcon-org/Falcon,falcon-org/Falcon