Add rebase script

This brings us in sync with the latest release more or less
automatically, if there are no merge conflicts that can't be
automatically resolved.

rebase.sh

@@ -0,0 +1,21 @@
+#!/bin/bash
+
+set -euxo pipefail
+
+OLD_TAG=$1
+NEW_TAG=$2
+
+git fetch upstream
+git fetch origin
+
+git reset --hard origin/main
+
+# this should fail, that's expected
+git rebase --onto $NEW_TAG $OLD_TAG $OLD_TAG-rust1 || true
+
+echo ${NEW_TAG/v/}-rust1 > src/runnerversion
+echo ${NEW_TAG/v/}-rust1 > releaseVersion
+echo "Fork of the GitHub Actions runner used by the Rust Infrastructure Team." > releaseNote.md
+git add src/runnerversion releaseVersion releaseNote.md
+git rebase --continue
+git push -f origin HEAD:main