catacomb-import-update: Handle deletion

catacomb-import-update: Handle deletion

If files are removed from the list to be processed, they should be
removed from the import directory, not left lying around stale.

(Files that disappear from Catacomb upstream would always produce an
error, and will still do so.)

This means we key off %changed.  If there is no entry in
%DONOR_REV_MAP then it means the file is being deleted; if the entry
is undef ie means the file is being added.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>