github actions: Fix test-one-commit when parent of head is merge commit

Instead of figuring out the commit *after* the last merge and rebasing on that
with a ~1 suffix, just figure out the last merge commit directly and rebase on
it. This way, if HEAD happens to be a merge commit, the rebase just succeeds
immediately without blank variables or errors.

From https://github.com/bitcoin/bitcoin/pull/28497#issuecomment-1743430631:

    The problem is that the PR only contains a one commit after the last merge,
    so the job _should_ be skipped, but the `pull_request.commits != 1` check
    is not smart enough to skip it because the PR is based on another PR and
    has merge ancestor commits. So specifically what happens is that after
    HEAD~ is checked out, the new HEAD is a merge commit, so the range `$(git
    log --merges -1 --format=%H)..HEAD` is equivalent to HEAD..HEAD, which is
    empty, so the `COMMIT_AFTER_LAST_MERGE` variable is empty and the rebase
    command fails.

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