fix repl rsearch with one-char query

The backward i-search needs to set the "start" parameter of rsearch
to the end (minus 1) of the current search window.
This fixes e.g.:
* searching backwards only one character remains stuck on the same match
* searching backwards 'aaa' skips one of the matches in 'aaaa'
Wo you were right @ivarne, thanks, and in the course of making it utf8-safe, I think I uncovered a bug in `rsearch`, cf. #9365.