largefiles: redo heads interception The largefiles extension replaces the "heads" wire command and tries to redirect all uses towards the custom "lheads" wire command. As seen in issue6384, this doesn't currently work for ssh. Instead of hooking into the _callstream interface, properly register the command for the peer instance and monkeypatch the executor to do the redirection. This works transparently for both all kinds of peers and both for the batch and non-batch case. Differential Revision: https://phab.mercurial-scm.org/D9663