mergestate: move most of of reset() into start() `ms.reset()` has somehow become a combination of two different things: 1. A constructor-like function creating an empty instance 2. 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. Differential Revision: https://phab.mercurial-scm.org/D9035

mergestate: make clean() only be about creating a clean mergestate This is similar to the previous patch, but moves the separation one step further out, to `merge.py`. Differential Revision: https://phab.mercurial-scm.org/D9034

mergestate: split up reset() for its two use cases We only have one place that calls `ms.reset()` with any arguments. That place is `mergestate.clean()`. The callers that call the function with no arguments seem to all just want delete the mergestate -- none of them look at the instance after calling `reset()`. Let's separate out the two different use cases to make the code clearer. I'll clean up further soon. Differential Revision: https://phab.mercurial-scm.org/D9033

mergestate: simplify reset(), knowing that `other` and `node` go together There's only one caller of `reset()` that passes any arguments at all, and that originates from `merge.py:1371`. That code always passes values for both `node` and `other`. Differential Revision: https://phab.mercurial-scm.org/D9032