mercurial loader fails on save code now

Save code now on staging [1] (or production [2]) for mercurial repositories triggers the following issues.

I did not investigate [3] further than that but on fresh staging (all deps up-to-date), triggering mercurial repositories save code now made sentry raise this issue (marked resolved as regression).

• [1] https://sentry.softwareheritage.org/share/issue/c5622cacc16141fcaace2a61eab1b3b1/
• [2] https://sentry.softwareheritage.org/share/issue/114c53a2e1104ba987d0771f846a78ad/
• [3] might be it's the loader altogether which is broken, i did not check

---

Migrated from T2750 (view on Phabricator)