dirstate: move the _dirfoldmap to dirstatemap...
dirstate: move the _dirfoldmap to dirstatemap
Now that dirstatemap is the source of truth for the list of directories, let's
move _dirfoldmap on to it.
This pattern of moving cached variables onto the dirstate map makes it easier to
invalidate them, as seen by how the cache invalidation functions are slowly
shrinking to just be recreating the dirstatemap instance.
Differential Revision:
https://phab.mercurial-scm.org/D983