Clients MUST send at least one "want" command in the request body.
Clients MUST NOT reference an id in a "want" command which did not
appear in the response obtained through ref discovery unless the
-server advertises capability `allow-tip-sha1-in-want`.
+server advertises capability `allow-tip-sha1-in-want` or
+`allow-reachable-sha1-in-want`.
compute_request = want_list
have_list