loader: Add scenarii around new visit with changes since last visit

Summary:
In this scenario, a visit has already taken place for the origin. So,
an existing snapshot exists.

But, this time, the PyPI project has changed and a new release has
been uploaded.

The visit must then result in a new snapshot.

The snapshot shares the same history as prior visit.

Related T421

To be rebased and merged after olasd's diffs (done once).

Test Plan: make test

Reviewers: olasd

Differential Revision: https://forge.softwareheritage.org/D416