fix: don't checkout target for strategy force

Don't checkout the `target`-branch for strategy `force` as it might not exist and as the checkout is unnecessary.

action.yml

@@ -40,6 +40,7 @@ runs:
         git config user.email ${{ inputs.git_committer_email }}
       shell: bash
     - name: Checkout ${{ inputs.target }}
+      if: inputs.strategy != 'force'
       run: git checkout ${{ inputs.target }}
       shell: bash
     - name: Merge ${{ inputs.source }} into ${{ inputs.target }}