tests: demonstrate crash caused by pinning of non-existent mergestate node
If the mergestate somehow contains a reference to a non-existent node,
it causes most commands to crash since
aaeccdb6e654 (repoview: pin
revisions for `local` and `other` when a merge is active,
2020-09-04). This patch adds a test showing that.
Differential Revision: https://phab.mercurial-scm.org/D9077
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true