rust-dirstatemap: properly decrement counter for tracked descendants
I found this bug when writing unit tests after the fact for the `DirstateMap`.
We never decremented the tracked descendants counter since we were always
resetting the node data before reading it. This also drops the use of `state`,
in favor of the new API to get that information.
Differential Revision: https://phab.mercurial-scm.org/D12431
(binary:application/octet-stream)