Add Timestamp git metadata file

[ngehrsitz]

This pull request improves upon #306.

It adds generation of a timestamp metadata file that can be used to tag builds. The output format can also be specified. This is useful when running parallel builds of a pipeline to determine which version is more up to date after pushing to a build storage like artifactory.

[jamieklassen]

Really excellent PR! I was able to test it out by using the following pipeline.yml:

---
resource_types:
- name: git
  type: registry-image
  source:
    repository: concourse/git-resource
    tag: pr-ubuntu-329
resources:
- name: ft
  type: git
  source:
    uri: https://github.com/concourse/ft
jobs:
- name: check-timestamp
  plan:
  - get: ft
    params:
      timestamp_format: ((format))
  - task: read-timestamp
    config:
      platform: linux
      image_resource:
        type: registry-image
        source: {repository: busybox}
      inputs:
      - name: ft
      run:
        path: cat
        args: ["ft/.git/commit_timestamp"]

so that doing stuff like fly -t dev sp -n -p manual-test -c pipeline.yml -v format=rfc gives me something like

% fly -t dev tj -w -j manual-test/check-timestamp
started manual-test/check-timestamp #7

Cloning into '/tmp/build/get'...
6d13a0d publish continuously
initializing
running cat ft/.git/commit_timestamp
Wed, 5 Aug 2020 12:13:29 -0400
succeeded

The tests are good, as is the documentation!