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
https://bz.mercurial-scm.org/1089
$ hg init
$ mkdir a
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ hg ci -m m a
$ mkdir a b
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ cd b
Relative delete:
$ hg ci -m m ../a
$ cd ..