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.
These docs should explain that, on the 2nd & subsequent calls, it actually _doesn't_ search the stored result set, and just assumes that the new match index is exactly one increment away from the previous index (modulo wraparound).