hooks: add some test about HGPLAIN setting and hooks
In Mercurial 5.7, hooks are now ran with HGPLAIN set, which is a behavior change
in. I could not find explicit test about it so I am adding one. The next
changesets will introduce more change to help user mitigate the behavior change
when needed.
Differential Revision: https://phab.mercurial-scm.org/D9979
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true