perl: make Git::IndexInfo work with SHA-256

perl: make Git::IndexInfo work with SHA-256

Most of the Git modules, git-svn excepted, don't know anything about the
hash algorithm and mostly work.  However, when we're printing an
all-zero object ID in Git::IndexInfo, we need to know the hash length.

Since we don't want to change the API to have that information passed
in, let's query the config to find the hash algorithm and compute the
right value.

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>