builtin/difftool: use parse_oid_hex

builtin/difftool: use parse_oid_hex

Instead of using get_oid_hex and adding constants to the result, use
parse_oid_hex to make this code independent of the hash size.

Additionally, correct a typo that would cause us to print one too few
characters on error, since we will already have incremented the pointer
to point to the beginning of the object ID before we get to printing the
error message.

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