Add Fixer class, which re-loads corrupt objects from origins

This is heavily inspired by https://archive.softwareheritage.org/swh:1:cnt:c6dda7698c6aecf71f744e4e4c01bc3e115db880;origin=https://forge.softwareheritage.org/source/snippets.git;visit=swh:1:snp:c2e3170b9927f3d356e120546b00f5a9d25a224c;anchor=swh:1:rev:fa9e387ba5e3470b93a5c5a8773ed598ef03d211;path=/vlorentz/analyze_consistency_failures.py
but reorganized to the database instead of ad-hoc text and pickle files.

Currently, this only implements recovering from Git origins.