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
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true