rust-status: don't trigger dirstate v1 rewrite when only v2 data is changed

The assumption that we need to rewrite (or append to) the dirstate if the
ignore pattern hash has changed or if any cached directory mtimes have changed
is only valid when using dirstate-v2. In dirstate-v1, neither of these things
are written to disk.
14 jobs for topic/stable/rust-hg-purge in 52 minutes and 25 seconds (queued for 7 seconds)