make CVS loader create one snapshot per visit

The CVS loader used to create one snapshot per loaded revision.
As pointed out by ardumont in D6745, this is wrong; Other loaders
create only one snapshot per visit.
Fix this issue and adjust tests expectations accordingly.

While here, show SHW IDs of loaded revisions and snapshots in regular
"info" log output, rather than only in "debug" log output. Previously,
only CVS-related data was shown at the "info" log level. Showing both
CVs and SWH data in log output is more informative.