Updater (cmdline): add synonym --site for 'upload' subcommand

The option --update-site is a little cumbersome to write. Let's have
a shortcut.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Updater (cmdline): add a 'diff' subcommand

The functionality was there for a long time, but only exposed through
the Swing UI of the updater so far.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Updater (cmdline): offer the different diff modes via options

We can show quite a couple of diff modes, let's have them exposed via
the command-line interface, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Updater (cmdline): fix imports

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Updater (cmdline): allow to resolve upload conflicts interactively

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