The 1.30.0 changes to backup directory structure had some bugs. This
commit fixes those.

* The find command for counting files per segment had an error in the
  regex that was previously hidden by each segment's files being in
  separate folders. We update the regex used by find to be correct in
  the case of multiple-digit content IDs.
* Parsing segment prefix fails if both the old and the new structures
  are present alongside each other and the desired backup is in the old
  structure. We update the function to accept timestamp and check the
  expected location of the specific backup being restored.