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.
https://bz.mercurial-scm.org/1089
$ hg init repo
$ cd repo
$ mkdir a
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ hg ci -m m a
$ mkdir a b
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ cd b
Relative delete:
$ hg ci -m m ../a
$ cd ..