branchcache: do not copy the `_dirty` flag
If the inherited branch cache is dirty, it will be written on disk, and the
super-set did not need to modify it, the on disk value for the subset will be
re-useable as is. So the super set does not needs to write the very same content
itself.
[workspace]
members = ["hg-core", "hg-cpython", "rhg"]
exclude = ["chg", "hgcli"]
resolver = "2"