help: add a build option for default hash

help: add a build option for default hash

We'd like users to be able to determine the hash algorithm that is the
builtin default in their version of Git.  This is useful for
troubleshooting, especially when we decide to change the default.  Add
an entry for the default hash in the output of git version
--build-options so that users can easily access that information and
include it in bug reports.

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