readline: skip escaping characters again

This is a minor performance improvement for readline. It skips to
escape individual characters again after escaping them before.

Signed-off-by: Ruben Bridgewater <ruben@bridgewater.de>

PR-URL: #41005
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>