remotefilelog: fix subtle bug I introduced in emitrevisions
Somehow in here we were hitting some cases where basenode was being
set to the same as node, which is Very Bad. This manifested as a rare
live-lock on the client as it looped forever waiting for the revision
to be fetched before trying to apply a delta to the same revision.
Differential Revision: https://phab.mercurial-scm.org/D4871