typing: add some type annotations to mercurial/phases.py
Some of these were helpful in typing other modules, and then I typed the
easy-ish ones. Black forces the long `Phasedefaults` definition to be wrapped,
which pytype seems OK with (as shown with `reveal_type()`), but it does seem to
confuse PyCharm a bit.
Differential Revision: https://phab.mercurial-scm.org/D10126
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true