bup-index learns to scrub out the deleted items

By removing the deleted items, the file bupindex is smaller and faster
to merge after each index updates.

Signed-off-by: Patrick Rouleau <prouleau72@gmail.com>
TMP: Show the deleted items when the option "-m" is used

This become useful when using --scrub, otherwise the deleted files keep
pilling up and we have no clue if they was just deleted or if they was a
long time ago.

Signed-off-by: Patrick Rouleau <prouleau72@gmail.com>