dirstatemap: add a common `_insert_entry` method for dirstatemap
This method is called to add a new DirstateItem to the map.
Each variant have a different implementation (which is … the point).
Differential Revision: https://phab.mercurial-scm.org/D11573
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.