Teach maven-helper to extract the GAV of the parent, too

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Introduce 'bump-pom-scijava --parent'

This subcommand updates the parent of ./pom.xml instead of pom-scijava
itself. This, too, is intended to be performed by a Jenkins job; it
requires bin/bump-pom-scijava.sh *and* bin/maven-helper.sh to be copied
into the respective Git checkout, though.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Be sure to use the correct remote in bump-pom-scijava

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
bump-pom-scijava requires a clean state

... i.e. no uncommitted changes

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