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.