filter-branch documentation: some more touch-ups. - The map function used to fail, but no longer does (since 3520e1e8687.) - Fix the "edge-graft" example. - Show the same using .git/info/grafts. Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at> Signed-off-by: Junio C Hamano <gitster@pobox.com>