show-index: fix uninitialized hash function

show-index: fix uninitialized hash function

In c8aed5e8da (repository: stop setting SHA1 as the default object
hash), we got rid of the default hash algorithm for the_repository.
Due to this change, it is now the responsibility of the callers to set
their own default when this is not present.

As stated in the docs, show-index should use SHA1 as the default hash
algorithm when run outside a repository. Make sure this promise is
met by falling back to SHA1 when the_hash_algo is not present (i.e.
when the command is run outside a repository). Also add a test that
verifies this behavior.

Signed-off-by: Abhijeet Sonar <abhijeet.nkt@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>