t5300: choose the built-in hash outside of a repo

t5300: choose the built-in hash outside of a repo

Right now, the built-in default hash is always SHA-1, but that will
change in a future commit.  Instead of assuming that operating outside
of a repository will always use SHA-1, look up the default hash
algorithm for operating outside of a repository using an appropriate
environment variable, which will always be correct.

Additionally, for operations outside of a repository, use the
DEFAULT_HASH_ALGORITHM prerequisite rather than SHA1.

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