Add a helper script to determine the commit for a give .jar file

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Build.sh: Do not modify mtimes unnecessarily

E.g. ij-minimaven.jar *wants* to be left alone; otherwise the timestamp
check that should determine whether to download a newer version would
fail always for Jenkins (who builds several times a day).

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