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 looks like this code assumes that both `state.marked` and `state.resultSet` are non-null, but there are cases where that's not true:

* If there are > 2000 matches in the file, `state.marked` will be true, but we'll still get called (and `state.resultSet` will be available).
* If the file is > 500 KB, `state.resultSet` will be null.  This still gets called when you do Find Next, though.