Previously the same flag was used, with its meaning based on whether the node
otherwise identifies a file tracked anywhere.
In addition to being more explicit, this enables storing a directory mtime
if a given path used to be tracked in a parent commit (so the dirstate still
has data about it) but became a directory in the working copy.
(However this is not done yet as it would require a larger change,
replacing the dirstate_map::NodeData enum with struct fields.)