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

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 swh-loader-core#421 (closed)

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

Test Plan

make test

Migrated from D416 (view on Phabricator)