ms.reset() has somehow become a combination of two different things:
- A constructor-like function creating an empty instance
- A call to shutil.rmtree() to clear the mergestate.
It seems that all callers now care only about the latter (since we
changed one caller to use the new ms.start() method instead). Let's
move the code for the former into start(), since that's the only
place it's needed.
Ahhh - OK. This is much better.