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'
You'll get even more confused if you look at https://github.com/JuliaLang/julia/issues/7811, or inadvertantly finds out that `SubString` behaves differently. I hope someone will fix that, (or clean up after my most likely failed attempt).