Revert "emacs: track "dirty" point, where contents start differing"
Revert "emacs: track "dirty" point, where contents start differing"

This reverts commit 1a34c1f2910a1e25bf9714aa565e0bba282afa47.
This fix was wrong, a bug in backend was shadowing the real problem.
An off-by-one problem remains, fix coming soon.

neojski/merlin,neojski/merlin,ocaml/merlin,neojski/merlin,fxfactorial/merlin,haselwarter/merlin,chriskonstad/merlin,dra27/merlin,rgrinberg/merlin,haselwarter/merlin,chambart/merlin,the-lambda-church/merlin,dra27/merlin,chriskonstad/merlin,fxfactorial/merlin,fxfactorial/merlin,the-lambda-church/merlin,fxfactorial/merlin,chambart/merlin,rgrinberg/merlin,the-lambda-church/merlin,rgrinberg/merlin,chambart/merlin,dra27/merlin,ocaml/merlin,chambart/merlin,ocaml/merlin,haselwarter/merlin,ocaml/merlin,neojski/merlin,rgrinberg/merlin,dra27/merlin,haselwarter/merlin