treediscovery: always return all remote heads
Old discovery only returned incoming heads, not all of them (for
changegroupsubset). New discovery must always return all of the remote heads
(for getbundle). I failed to properly adjust treediscovery in cb98fed52495
when introducing setdiscovery.
The actual observable problem was 'remote: unsynced changes' when trying
to push a cset on one named branch to a server with a new cset on another
named branch. This scenario is now tested in test-treediscovery.t.