filter-branch: also don't fail in map() if a commit cannot be mapped

filter-branch: also don't fail in map() if a commit cannot be mapped

The map() function can be used by filters to map a commit id to its
rewritten id. Such a mapping may not exist, in which case the identity
mapping is used (the commit is returned unchanged).

In the rewrite loop, this mapping is also needed, but was done
explicitly in the same way. Use the map() function instead.

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>