builtin/ls-tree: let the oid determine the output algorithm

builtin/ls-tree: let the oid determine the output algorithm

Update cmd_ls_tree to call get_oid_with_context and pass
GET_OID_HASH_ANY instead of calling the simpler repo_get_oid.

This implments in ls-tree the behavior that asking to display a sha1
hash displays the corrresponding sha1 encoded object and asking to
display a sha256 hash displayes the corresponding sha256 encoded
object.

This is useful for testing the conversion of an object to an
equivlanet object encoded with a different hash function.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>