rust-chg: reimplement ChgClientExt as ChgClient wrapper
ChgClient is no longer an extension trait because:
a. Client object is not consumed and recreated in future-0.3 world, which
unblocks writing a simple wrapper struct.
b. async fn isn't allowed in trait.
Overall, the API should become simpler.
Differential Revision: https://phab.mercurial-scm.org/D8446
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true