bump-pom-scijava: handle more than the first default property

We cannot use 'set' to override the first two parameters because the
rest would be forgotten...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
bump-pom-scijava: work around 'git diff --quiet --no-index' bug

The exit status is not set correctly... simply use 'cmp' instead.

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