mergestate: move commit() from base class to on-disk subclass

The commit message doesn't do anything in the in-memory case, and it's
_makerecords() is also not needed in that case, so let's move it
down to the on-disk subclass (i.e. the mergestate class). Sorry I
missed this earlier.

Differential Revision: https://phab.mercurial-scm.org/D9043