t9350: make hash size independent

t9350: make hash size independent

This test checks for several commit object sizes to verify that objects
are encoded as expected. However, the size of a commit object differs
between SHA-1 and SHA-256, since each contains a hex representation of
the tree's object ID. Since these are root commits, compute the size of
each commit by using a constant plus the size of a single hex object ID.

In addition, use $ZERO_OID instead of a hard-coded object ID.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>