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
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2