Allows pushing to a branch with dynamic name

[wrighbr]

Allows pushing to a branch with a dynamic name this relates to issue #376

takes an output file from a task branchname and using this in params.branch

  - task: version-bump
    config:
      platform: linux
      image_resource:
        type: registry-image
        source:
          repository: node
          tag: 16.15-buster
      inputs: 
        - name: repo
      outputs:
        - name: repo
      run:
        path: sh
        args: 
          - -ec
          - |
            cd repo
   
            echo updating-to-v${NEW_VERSION}-$(date +%Y%m%d) > branchname
   
            git config user.email [email protected]
            git config user.name concourse

            echo $(date +%Y%m%d) date

            git add date
            git commit -m "updating repo"

  - put: repo-update
    params:
      repository: repo
      branch: repo/branchname