Checkout action is undoing some repo history

[pcolmer]

I'm checking out a repository thus:

      - name: Fetch git repository
        uses: actions/checkout@v2
        with:
          fetch-depth: 0
          ssh-key: ${{ secrets.LINARO_BUILD_KEY }}
          path: website

However, when the repo is fetched, the action does the following:

  /usr/bin/git rev-parse refs/remotes/origin/master
  d84ba4322bb75f2aa391d691680fc829293443ac
  /usr/bin/git -c protocol.version=2 fetch --no-tags --prune --progress --no-recurse-submodules origin +a3a4c887009c42b68bdb359949807ea66deccf77:refs/remotes/origin/master
  From github.com:Linaro/website
   + d84ba432...a3a4c887 a3a4c887009c42b68bdb359949807ea66deccf77 -> origin/master  (forced update)

thus undoing several commits in the repo. Why is it doing this, please? Is there a setting I'm overlooking? I don't know where the action is getting the commit reference a3a4c887009c42b68bdb359949807ea66deccf77 from but I need it to stop doing it!

[pcolmer]

Please ignore the noise ... the commit reversion was happening because I was re-running a previously failed job, rather than running the workflow "today" afresh.