This is a follow up to change
I8e4e5afc773d53dee9c1c24951bb07a45ddc2f1a which fixed an issue with
validation when the topmost patch after a Zuul rebase is a merge
patch.
We need to also use the $commit_hash variable for the check for
stable-only patches, else it will incorrectly fail because it is
checking the merge patch's commit message.
Change-Id: Ia725346b65dd5e2f16aa049c74b45d99e22b3524
1e10461c71