dirstatemap: move `_dirs_incr` and `_dirs_decr` methods out of the common
They are only used by the Python implementation now
Differential Revision: https://phab.mercurial-scm.org/D12516
dirstatemap: move `_refresh_entry` out of the common methods
This is only used in the Python implementation now
Differential Revision: https://phab.mercurial-scm.org/D12515
dirstatemap: move `_drop_entry` out of the common methods
Only the Python implementation uses it.
Differential Revision: https://phab.mercurial-scm.org/D12514