Do not try to extend last match when updating regex matches
Do not try to extend last match when updating regex matches

It does not work well with regexes starting with a lookbehind, as
we would need to reparse from further away, leading to the last
match just being removed. It seems safer not to remove it, as
the motivating use case (multiline macros) is better left to
regions anyway.

Fixes #440

jkonecny12/kakoune,danr/kakoune,ekie/kakoune,casimir/kakoune,lenormf/kakoune,alpha123/kakoune,alexherbo2/kakoune,jjthrash/kakoune,ekie/kakoune,jjthrash/kakoune,casimir/kakoune,Somasis/kakoune,alexherbo2/kakoune,jkonecny12/kakoune,mawww/kakoune,alexherbo2/kakoune,flavius/kakoune,alpha123/kakoune,occivink/kakoune,ekie/kakoune,flavius/kakoune,jkonecny12/kakoune,alexherbo2/kakoune,Somasis/kakoune,jkonecny12/kakoune,mawww/kakoune,alpha123/kakoune,alpha123/kakoune,Somasis/kakoune,casimir/kakoune,occivink/kakoune,jjthrash/kakoune,lenormf/kakoune,lenormf/kakoune,danr/kakoune,ekie/kakoune,jjthrash/kakoune,lenormf/kakoune,Somasis/kakoune,flavius/kakoune,occivink/kakoune,mawww/kakoune,casimir/kakoune,occivink/kakoune,danr/kakoune,danr/kakoune,flavius/kakoune,mawww/kakoune