Adapt the git loader to swh-model >0.4
in which extra_headers is now a top-level attribute of a Revision.
Add tests that actually include having extra_header in loaded git revisions. For this, the fast-import based dump of the test git repo is replaced by a bundle dump (required to ensure things like gpg signatures are kept). A few git revisions including gpg signature, encoding and mergetag headers are also added to the git repo.
Migrated from D3454 (view on Phabricator)