Add .git/branch as additional populated file

[jmatsushita]

Would you consider adding a .git/branch file in the additionally populated files with the branch name? Unless I'm missing something I can't see how to transform the branch name that triggered the pipeline into a variable for instantiated pipelines.

This would be nice to make use of the instanced pipeline experimental feature and use set_pipeline with the branch value in this way:

- name: configure-pipelines
  public: true
  plan:
    - get: my-repo
      trigger: true

    - load_var: branch_name
      file: .git/branch

    - set_pipeline: build
      file: my-repo/.concourse.yml
      instance_vars:
        branch: ((.:branch_name))

The implementation could be

git symbolic-ref -q --short HEAD > .git/branch

[jmatsushita]

I see it was discussed in this PR #257

Another implementation from that discussion:

git branch --remote --contains HEAD --format="%(refname:lstrip=-1)" > .git/branch

[n-insaidoo]

Isn't there anyway you could reference the branch attribute from the resources object elsewhere in the configuration?