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