git-svn: set the OID length based on hash algorithm

git-svn: set the OID length based on hash algorithm

When reading the configuration or when creating a new repository, load
the extensions.objectFormat value and set the object ID length to 64 if
it's "sha256".  Note that we use the hex length in git-svn because most
of our processing is done on hex values, not binary ones.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Acked-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>