Add new script to determine previous tag

Old script used hard coded release branch, new one uses tag versions directly. This is to address the fact that we currently tag v0.9.X and v0.10.X at the same time. That confused the previous incantation of the incremental build job.

CI/incremental_build_setup.sh

-current_tag=$(git describe --exact-match HEAD 2>&1)
-if [ "$?" -ne 0 ]; then
-  echo "Not on tagged commit"
-  # use previous tag on branch
-  export prev_tag=$(git describe --tags --abbrev=0 origin/release)
-else
-  echo "On tagged commit $current_tag"
-  # this is the case if we are on the release branch
-  # use tag before this tag
-  export prev_tag=$(git describe --tags --abbrev=0 origin/release~)
-fi
-
-echo "Previous tag to build first $prev_tag"