rust-dirstatemap: inline the last `get_or_insert` call The `get_or_insert` method was dangerous because it did not take the `DirstateMap` counters into account. This particular instance does not need to update the counters. Differential Revision: https://phab.mercurial-scm.org/D12527

rust-dirstatemap: add `set_cached_mtime` helper method This will help remove the `get_or_insert` method, which is dangerous because it does not take the `DirstateMap` counters into account Differential Revision: https://phab.mercurial-scm.org/D12526