Updater: allow reinstating previously obsolete files

Noticed by Curtis in Ticket #1603.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
UpdaterTest: remove temporary directories after successful test

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
UpdaterTest: try to prevent erroneous use of writeJar()

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'master' of git://github.com/imagej/imagej