discovery: diet discovery.prepush from non-discovery code The ``discovery.prepush`` function was doing multiple things not related to discovery. This changeset move some code into the ``localrepo.push`` method. The old ``discovery.prepush`` function jobs is now restricted to checking for multple head creation. It was then renamed ``discovery.checkheads``. This new ``discovery.checkheads`` function may receive several other changes in the future but we are a bit too much near the freeze for a wider refactoring.