In some cases where `redisplay()` is called immediately after a
command is run (e.g. `log ...`) we often get a prompt, junk output,
and a second prompt.  This is due to a (known) race.

We believe we're falling afoul of this code in `el_deletestr`:

    if (el->el_line.cursor < &el->el_line.buffer[n])
            return;

Basing the length of text to delete off of the cursor position
resolves the issue of junk text, but the real solution is to eliminate
the race conditions, which will also resolve the sometimes duplicated
prompt.

FS-6764 #resolve

Thanks-to: Nathan Neulinger <nneul@neulinger.org>