Enhance the safety of the -U option:

Enhance the safety of the -U option:
1. In several places make sure that the mtree database is not empty using
the -s argument to test instead of -f. (I thought I'd already changed this,
but obviously not.)
2. When deriving the list of changed files use a colon-delimited list.
3. If the list of changed files is empty, unset the variable, and test
for a non-empty CHANGED in diff_loop() before entering that routine.

Enhance the speed of the -U option by using an internal case statement
instead of echoing the variable to an external grep for every file.

Fix indentation on one line.