branchcache: change the _delayed flag to an explicit `_dirty` flag This is more consistent with the logic we use for other object and it open the way to a clearer management of the cache state. Now, cache are created clean, cache update mark them dirty, writing them on disk mark them clean again.

[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