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>
Updater (cmdline): allow to resolve update conflicts interactively, too

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'updater-cmdline'

This merge brings a couple of improvements to the command-line version
of the updater, most notably the 'diff' subcommand and the ability of
resolving conflicts interactively (if an interactive console is
connected).

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