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
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed