gh-123177: Fix prompt for wrapped lines in pyrepl (#123324)

gh-123177: Fix prompt for wrapped lines in pyrepl (#123324)

When display lines above the cursor come from the cache, the first line
to not come from the cache may be a wrapped line, starting half way
through a logical line in the buffer. Detect and handle this case to
avoid accidentally drawing a stray prompt in the middle of a logical
line.