Completion should handle combining accents equivalents

[Samuel Thibault]

Hello,

This is tested with libreadline 8.0 on Debian.

If I create a directory named héhé where é is e + U+0301
(combining acute accent), and then try to complete hé into it where é
is the precombined form U+00E9), libreadline does not take care that é
(e with combining acute accent) is equivalent to é (the precombined "e
with acute accent" character), and fails to complete.

This is a large concern when dealing with zip files etc. coming from
systems that use decomposed forms, such as Apple systems.

Samuel