Fixes regression introduced by my fix for #16847: we would not receive
any alert when this cronjob fails.

Accordingly, I'm removing the -x flag passed to "git clean", otherwise we would
delete the .ikiwiki/ directory, which contains the information ikiwiki needs
to refresh. This has other side effects, such as leaving POT files around;
if that's a problem, we may need to delete them (without using "git clean")
in save-suggestions.py.

All the PIP packages Weblate 2.20 needs are in Buster.

Some of these in Buster are new enough for Weblate 3.0.

This should not matter here but let's not take risks.

At least filelock can't be installed without setuptools.

This will avoid this cronjob sending us a daily email with warnings.

It conflicts with --rebuild which overrides it.

I've tried --refresh without --rebuild and that fails because
save-suggestions.py does "git clean -fdx", which deletes the files ikiwiki needs
to know what to refresh.

The "-" prefix is deprecated.

Time-based APT snapshots: stop tracking the obsolete obfs4proxy dist at deb.torproject.org (#16848).

Tails stopped using this dist at 5e5cb9179e5cc9c479ce90f8ea6fd7dfb7d3bf03.

The Tor project stopped publishing this dist.

Manage langs.json on the Git server: it's needed by the update.secondary hook that sanity checks pushes by Weblate.

Previously we cleaned it up only on errors.

And in passing, rename to "cron" the "tails" remote dedicated to our cronjobs,
in the hope it's a tiny bit less ambiguous.

Ensure we merge the commit we know we can safely fast-forward to, instead of whatever the remote branch has become since we did this check.