bug#48535: 28.0.50; ff-find-other-file no longer sets current-buffer?

[Stephen Leake <stephen_leake@stephe-leake.org>]

This breaks emacs ada-mode automated tests, which rely on evaluating
elisp forms in Ada comments.

git bisect says this commit is to blame:

commit: c105017c44d4a679f7af739b2c0390b2c7850569
Author: Stefan Monnier <monnier@iro.umontreal.ca>
Date:   Tue Apr 6 17:06:07 2021 -0400

    * lisp/find-file.el: Make the commands oblivious to mouse/non-mouse

    (ff-find-other-file): Add `event` argument.
    (ff-find-other-file-other-window): Rename from
    `ff-mouse-find-other-file-other-window` and use this new argument.
    (ff-mouse-find-other-file, ff-mouse-find-other-file-other-window):
    Make them obsolete aliases.
    (ff-upcase-p): Remove unused `start` and `end` arguments and
    simplify accordingly.

I looked thru the diff, but I didn't see anything obvious, so I'm
blaming the lexical-binding change. But I don't know how to fix it.

I've confirmed that disabling lexical-binding fixes the problem.

-- 
-- Stephe