Show match index in place of match count

This is the implementation of https://trello.com/c/0GXqt5GF/1089-s-find-next-indicate-current-match-index.
It would be good to document here (or somewhere) that this is a _1-based_ index, since that's unusual.  (Actually, I'd prefer if it was a 0-based index, but that's probably not a blocker as long as the var name or docs make it very clear).