migrating tar restore to use the same code as index restore

migrating tar restore to use the same code as index restore

previous commit was a mess because it mixed two commits, my fault.
it mixed both the mtime directories fix and the generalization of
tar restore with index restore.

this commit continues the work towards this generalization, fixing
the TarPathIterator (which was just not tested), doing the removed
os.chdir in restore_backup (which resulted in cwd data loss when
running the tests), fixing also the indentation at the end of the
restore_backup function, and setting correctly the
new_volume_handler in RestoreHelper when restoring from a tarball.

However, there are still some unit tests related to multivolume
handling that still fail.