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'
Great! Julia makes UTF-8 standard, and does it right, but makes it somewhat too easy to make bugs like this. Maybe #9297, and hiding the efficient interface behind a wrapped type has something to it anyway?