builtin: use default hash when outside a repository

builtin: use default hash when outside a repository

We have some commands that can operate inside or outside a repository.
If we're operating outside a repository, we clearly cannot use the
repository's hash algorithm as a default since it doesn't exist, so
instead, let's pick the default instead of specifically SHA-1.  Right
now this results in no functional change since the default is SHA-1, but
that may change in the future.

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