Fix GitLoaderFromArchive.

Summary:
SWHLoader calls prepare(); and if prepare() fails, it calls cleanup().
However, GitLoaderFromArchive.cleanup() uses self.temp_dir and self.repo_path,
so they have to be initialized.

Additionally, GitLoaderFromArchive.prepare_origin_visit does not accept
the same argument name as its parent, so it fails when called by
GitLoaderFromArchive.load.

Reviewers: Reviewers, olasd

Reviewed By: Reviewers, olasd

Subscribers: swh-public-ci

Differential Revision: https://forge.softwareheritage.org/D621