release-version --tag: exit early on existing tag

It can be really, really annoying when the quite long process of testing
and retesting a release candidate is done, successfully, only so that
release-version.sh can complain about an existing tag.

So let's not do that. (Fail early, fail often!)

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

release-version.sh

@@ -34,6 +34,8 @@ do
 	--skip-push) SKIP_PUSH=t;;
 	--skip-deploy) SKIP_DEPLOY=t;;
 	--tag=*)
+		! git rev-parse --quiet --verify refs/tags/"${1#--*=}" ||
+		die "Tag ${1#--*=} exists already!"
 		TAG="-Dtag=${1#--*=}";;
 	--dev-version=*|--development-version=*)
 		DEV_VERSION="-DdevelopmentVersion=${1#--*=}";;