loader: Yield only modified objects in process_cvs_changesets

Previously, after each revision replay all files and directories of the
CVS repository being loaded were collected and sent to the storage.
This is a real bottleneck in terms of loading performances as it delegates
the filtering of new objects to archive to the storage filtering proxy.

As we known exactly the set of paths that have been modified in a CVS
revision, prefer to do that filtering on the loader side and only
send modified objects to storage instead of the whole set of contents
and directories from the reconstructed filesystem.

This should greatly improve loading performance for large repositories
but also reduce loader memory consumption.