manifest: introduce new exception to signal unavailability of fastdelta()
I've spent some time reflecting on this, and I think this is the best
we can do in this API contract for now. This opens the door to adding
treemanifest's implementation to the list of implementers of
imanifestdict.
Differential Revision: https://phab.mercurial-scm.org/D8365
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true