This move contains the first reference to debugrevlogopts in
debugcommands.py. We'll eventually want to move that over. We
hold off for now because it would introduce a module import cycle.
commands.py is our largest .py file by nearly 2x. Debug commands live
in a world of their own. So let's extract them to their own module.
We start with "debugancestor."
We currently reuse the commands table with commands.py and have a hack
in dispatch.py for loading debugcommands.py. In the future, we could
potentially use a separate commands table and avoid the import of
debugcommands.py.
