Fix overly strict backward_sorted_until
Fix overly strict backward_sorted_until

A change that ended exactly where the previous one started was not
considered backward sorted. Leading to some very bad performances in
certain cases, like '100000o<esc>u'

jjthrash/kakoune,casimir/kakoune,danr/kakoune,jkonecny12/kakoune,mawww/kakoune,lenormf/kakoune,jkonecny12/kakoune,ekie/kakoune,casimir/kakoune,danr/kakoune,lenormf/kakoune,Somasis/kakoune,lenormf/kakoune,ekie/kakoune,jjthrash/kakoune,alexherbo2/kakoune,Somasis/kakoune,occivink/kakoune,occivink/kakoune,mawww/kakoune,ekie/kakoune,danr/kakoune,mawww/kakoune,danr/kakoune,occivink/kakoune,jkonecny12/kakoune,jkonecny12/kakoune,casimir/kakoune,lenormf/kakoune,alexherbo2/kakoune,Somasis/kakoune,Somasis/kakoune,alexherbo2/kakoune,jjthrash/kakoune,occivink/kakoune,jjthrash/kakoune,alexherbo2/kakoune,mawww/kakoune,casimir/kakoune,ekie/kakoune