svn.loader: Simplify and align incremental visit algorithm...
... with other loaders.
Prior to this commit, the pre-snapshot existing code to resolve the starting point of a visit was convoluted and hard to follow.
This simplifies it to actually always use the snapshot to start a new visit as other loaders do.
This also:
- adds more type on impacted methods
- rework slightly docstrings on impacted methods
- adds more coverage on the remote dump loader implementation
- fixes an
eventful
assertion which should have beenuneventful
Test Plan
tox
Migrated from D3828 (view on Phabricator)