Add upstream helper script

[ci skip]

scripts/fetch-upstream

+#!/bin/bash
+
+REMOTE=upstream
+REMOTE_URL=https://github.com/Icinga/deb-icinga2.git
+
+if ! git config "remote.${REMOTE}.url" &>/dev/null; then
+  git remote add "${REMOTE}" "${REMOTE_URL}"
+elif [ "$(git config "remote.${REMOTE}.url")" != "${REMOTE_URL}" ]; then
+  git remote set-url "${REMOTE}" "${REMOTE_URL}"
+fi
+
+if [ "$(git config "remote.${REMOTE}.tagOpts")" != "--no-tags" ]; then
+  git config "remote.${REMOTE}.tagOpts" "--no-tags"
+fi
+
+git fetch "${REMOTE}"