main: Code cleanup by passing OstreeRepo * directly to builtins

It turns out every builtin (with one special exception) that takes a
repo argument did the same thing; let's just centralize it.  The
special exception was "ostree init --repo=foo" where foo is expected
to *not* actually be a repo.  In that case, simply skip the
ostree_repo_check() invocation.

https://bugzilla.gnome.org/show_bug.cgi?id=706762

src/ostree/ot-builtin-checksum.c

@@ -56,7 +56,7 @@ on_checksum_received (GObject    *obj,
 }
 
 gboolean
-ostree_builtin_checksum (int argc, char **argv, GFile *repo_path_path, GCancellable *cancellable, GError **error)
+ostree_builtin_checksum (int argc, char **argv, OstreeRepo *repo, GCancellable *cancellable, GError **error)
 {
   GOptionContext *context;
   gboolean ret = FALSE;