[meta] attempt to add an autorebase Github action

Author: ljharb

.github/autorebase.workflow

@@ -0,0 +1,9 @@
+workflow "Autorebase on merge commits" {
+  on = "push"
+  resolves = ["docker://ljharb/rebase:latest"]
+}
+
+action "docker://ljharb/rebase:latest" {
+  uses = "docker://ljharb/rebase:latest"
+  secrets = ["GITHUB_TOKEN"]
+}