1. Update the message that prints out for -U when the mtree database does

1. Update the message that prints out for -U when the mtree database does
not exist to let the user know that it will be created for the next run.

2. Delete more stuff we're not going to use from the temproot prior to
creating the mtree database to dramatically reduce its size (162K -> 37K).

3. We've been deleting the zero-size files from temproot for a long time
now, so remove the spurious "-size +0" from the find command in the
comparison loop, and remove what is now a really stale comment.