bug#66187: read-file-name unexpected behavior when MUSTMATCH is a function

[Drew Adams]

> Since completing-read is such a fundamental part of Emacs,
> I doubt it will be possible to change this behavior.

Apparently it was possible for Someone (TM) to
change the behavior, in spite of the fact that
it's a longstanding "fundamental part of Emacs".
It _was_ a fundamental part of Emacs, but it
seems that somehow it's signature was changed
for Emacs 29.

The new behavior for a function value of
REQUIRE-MATCH is backward-incompatible.

A function as REQUIRE-MATCH value _should_ still
be treated as any other value that's not nil, t,
`confirm', or `confirm-after-completion'.  I
filed bug #66328 for this.