check-release: avoid using maven-helper to determine latest version

It's pretty expensive, what with many roundtrips to our Nexus. Besides,
we have the tags locally anyway.

But let's have the --ask-nexus-for-latest-version option to force the
previous behavior.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
check-release.pl: do not update local repository by default

... but offer the previous behavior via --git-fetch-first.

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