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
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 ..