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
#require docutils gettext
$ $TESTDIR/check-gendoc zh_CN
checking for parse errors