When the `bullseye.mdwn` page was renamed (to `bullseye.md`) in this
commit:

commit 6fe448dc
Author: Antoine Beaupré <anarcat@debian.org>
Date:   Wed Sep 1 11:33:12 2021 -0400

    rename bullseye upgrade procedure file

    This is the only .mdwn file out there, strangely. Let's hope this is
    going to fix the garbled display issues, where code blocks show up
    with <pre><code> blobs.

diff --git a/howto/upgrades/bullseye.mdwn b/howto/upgrades/bullseye.md
similarity index 100%
rename from howto/upgrades/bullseye.mdwn
rename to howto/upgrades/bullseye.md

... the CI failed, because the hook couldn't find the `.mdwn` file to check:

https://gitlab.torproject.org/tpo/tpa/wiki-replica/-/jobs/35296

The actual error was:

$ ./bin/mdl-wrapper $(cat changed-files.txt)
checking file howto/upgrades/bullseye.md...
checking howto/upgrades/bullseye.mdwn...
/usr/lib/ruby/gems/2.6.0/gems/mdl-0.11.0/lib/mdl/doc.rb:50:in `read': No such file or directory @ rb_sysopen - howto/upgrades/bullseye.mdwn (Errno::ENOENT)
	from /usr/lib/ruby/gems/2.6.0/gems/mdl-0.11.0/lib/mdl/doc.rb:50:in `new_from_file'
	from /usr/lib/ruby/gems/2.6.0/gems/mdl-0.11.0/lib/mdl.rb:82:in `block in run'
	from /usr/lib/ruby/gems/2.6.0/gems/mdl-0.11.0/lib/mdl.rb:80:in `each'
	from /usr/lib/ruby/gems/2.6.0/gems/mdl-0.11.0/lib/mdl.rb:80:in `run'
	from /usr/lib/ruby/gems/2.6.0/gems/mdl-0.11.0/bin/mdl:10:in `<top (required)>'
	from /usr/bin/mdl:23:in `load'
	from /usr/bin/mdl:23:in `<main>'
Cleaning up file based variables 00:03
ERROR: Job failed: exit code 1

In our code, we assume that if a path is not a file, it's a directory,
but that's not quite right: it could also be just missing!

So handle that other case more elegantly.