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 <[email protected]>

PR-URL: #41005
Reviewed-By: Michaël Zasso <[email protected]>
Reviewed-By: Benjamin Gruenbaum <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Anna Henningsen <[email protected]>