github: pass --rebase to gh pr merge since it requires it

It seems that there's no way to just enable auto-merge without
specifying a merge strategy (presumably because some projects allow
several GitHub merge strategies), so I guess we'll have to live with
the strategy being duplicated between here and the project settings.

.github/workflows/dependabot.yml

@@ -12,7 +12,7 @@ jobs:
     if: ${{ github.actor == 'dependabot[bot]' }}
     steps:
     - name: Enable auto-merge for Dependabot PRs
-      run: gh pr merge --auto "$PR_URL"
+      run: gh pr merge --auto --rebase "$PR_URL"
       env:
         PR_URL: ${{github.event.pull_request.html_url}}
         GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}