maven-helper: accelerate tag extraction dramatically

It is amazing how much faster it is to match greedily...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Do not try to extract a parent GAV when there is no parent

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
bump-parent: be nice when there is no parent to bump

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