py3: return False early while checking whether None is a key in lazymanifest In this patch we returns False early if we are trying to check whether None is a key in lazymanifest. The reason I added a diff is that on Python 3, it goes into the lazy manifest code an returns a TypeError. I checked with Durham that whether None can be a possible key in lazymanifest and he said "no". So it's safe to have this if statement. This fixes `hg merge` on Python 3. Differential Revision: https://phab.mercurial-scm.org/D852