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'
Is this Unicode safe, or should we rather use `nextind` and `prevind`?