repository: make cleanup safe for re-use with grafts

We are allowed to call `git_repository__cleanup` multiple times, and this
happens e.g. in rugged if we want to free up resources before GC gets around to
them.

This means that we cannot leave dangling pointers in it, which we were doing
with the grafts. Fix this by setting the pointers to NULL after freeing the resources.

src/libgit2/repository.c

@@ -153,7 +153,9 @@ int git_repository__cleanup(git_repository *repo)
 	git_cache_clear(&repo->objects);
 	git_attr_cache_flush(repo);
 	git_grafts_free(repo->grafts);
+	repo->grafts = NULL;
 	git_grafts_free(repo->shallow_grafts);
+	repo->shallow_grafts = NULL;
 
 	set_config(repo, NULL);
 	set_index(repo, NULL);