dirstatemap: create `_dirs_incr/_dirs_decr` methods on the common class...
dirstatemap: create `_dirs_incr/_dirs_decr` methods on the common class
The Rust wrapper does not need them. However having a default, no-op,
implementation will help use to write code used by both implementation.
Differential Revision:
https://phab.mercurial-scm.org/D11570
