Command-line updater: add 'list-shadowed' command

In the process of updating ImageJ2 components, it became apparent that a
couple too many files of the ImageJ update site were shadowed by the Fiji
update site.

The code to identify these files is too useful to keep from others; hence
this commit.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>