filter-branch: Avoid an error message in the map function.

filter-branch: Avoid an error message in the map function.

When the map function didn't find the rewritten commit of the passed in
original id, it printed the original id, but it still fell through to
the 'cat', which failed with an error message.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>