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

[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true