t: make SHA1 prerequisite depend on default hash

t: make SHA1 prerequisite depend on default hash

Currently, the SHA1 prerequisite depends on the output of git
hash-object.  However, in order for that to produce sane behavior, we
must be in a repository.  If we are not, the default will remain SHA-1,
and we'll produce wrong results if we're using SHA-256 for the testsuite
but the test assertion starts when we're not in a repository.

Check the environment variable we use for this purpose, leaving it to
default to SHA-1 if none is specified.

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>