This also allows the new extra_headers conversion.
(current revisions might not have it yet but at some point, they will eventually get migrated to this [1] ;)
That may also fix the master build now (some new code got pushed in the repository since the new storage 0.14.0 release) [2]
[1] T2564
[2] https://jenkins.softwareheritage.org/job/DWAPPS/job/tests/1631/console
Related to T645