main: Move GCancellable to toplevel

This is just cleaner, and makes the builtins slightly more of a
library, which in turn makes it easier to turn them into real API.

src/ostree/ot-builtin-checksum.c

@@ -53,7 +53,7 @@ on_checksum_received (GObject    *obj,
 }
 
 gboolean
-ostree_builtin_checksum (int argc, char **argv, GFile *repo_path_path, GError **error)
+ostree_builtin_checksum (int argc, char **argv, GFile *repo_path_path, GCancellable *cancellable, GError **error)
 {
   GOptionContext *context;
   gboolean ret = FALSE;
@@ -77,7 +77,7 @@ ostree_builtin_checksum (int argc, char **argv, GFile *repo_path_path, GError **
 
   data.loop = g_main_loop_new (NULL, FALSE);
   data.error = error;
-  ostree_checksum_file_async (f, OSTREE_OBJECT_TYPE_FILE, G_PRIORITY_DEFAULT, NULL, on_checksum_received, &data);
+  ostree_checksum_file_async (f, OSTREE_OBJECT_TYPE_FILE, G_PRIORITY_DEFAULT, cancellable, on_checksum_received, &data);
   
   g_main_loop_run (data.loop);