checkout: use leak_pending flag

checkout: use leak_pending flag

Instead of going through all the references again when we clear the
commit marks, do it like bisect and bundle and gain ownership of the
list of pending objects which we constructed from those references.

We simply copy the struct object_array that points to the list, set
the flag leak_pending and then prepare_revision_walk won't destroy
it and it's ours.  We use it to clear the marks and  free it at the
end.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>