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

dirstatemap: small rework of the `set_untracked` method This shuffle the code a bit to have it flowing more "naturally". This will help us to create a common version of this code in the next changesets. Differential Revision: https://phab.mercurial-scm.org/D11569