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'
Really I never had to think on utf8 things before today, but I already noticed that I don't like the `getindex` interface with `Int`, with invalid positions,  and there are inconsistencies, like: `a="éé"; a[1:1] == a[1:2]; a[3] == a[3:4]`: the last indexing is a `BoundsError`, but not `a[1:2]`, I'll read the thread you linked to, thanks.