histedit: use new function for getting first line of a string
This fixes a crash you can run into if you enter a commit message
that's just a "newline-like" byte, like a form feed byte (`hg ci -m
\x0f` in Fish). That bug is the motivation for this series.
Differential Revision: https://phab.mercurial-scm.org/D12405
[build-system]
requires = ["setuptools", "wheel"]
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true