add git_dir option to git module to allow external repository dir

Tested using the below playbook, using one repository without submodules, and one with.

```yaml
---
- name: reset repositories
  command: rm -rf /home/ansible/git

- name: fetch repositories without git_dir
  git: >
    repo=https://github.com/{{ item[0] }}.git
    version={{ item[1] }}
    dest=/home/ansible/git/combined/{{ item[0] }}
  with_nested:
    - ['joyent/http-parser', 'ShareKit/ShareKit']
    - ['HEAD', 'v2.3', 'v2.0']

- name: fetch repositories with git_dir
  git: >
    repo=https://github.com/{{ item[0] }}.git
    version={{ item[1] }}
    dest=/home/ansible/git/work_tree/{{ item[0] }}
    git_dir=/home/ansible/git/git_dir/{{ item[0] }}.git
  with_nested:
    - ['joyent/http-parser', 'ShareKit/ShareKit']
    - ['HEAD', 'v2.3', 'v2.0']

- name: reset repositories
  command: rm -rf /home/ansible/git

- name: fetch repositories without git_dir
  git: >
    repo=https://github.com/{{ item[0] }}.git
    version={{ item[1] }}
    dest=/home/ansible/git/combined/{{ item[0] }}
  with_nested:
    - ['joyent/http-parser', 'ShareKit/ShareKit']
    - ['v2.3', 'HEAD', 'v2.0']

- name: fetch repositories with git_dir
  git: >
    repo=https://github.com/{{ item[0] }}.git
    version={{ item[1] }}
    dest=/home/ansible/git/work_tree/{{ item[0] }}
    git_dir=/home/ansible/git/git_dir/{{ item[0] }}.git
  with_nested:
    - ['joyent/http-parser', 'ShareKit/ShareKit']
    - ['v2.3', 'HEAD', 'v2.0']

- name: git_dir and bare defined together
  git: >
    repo=https://github.com/{{ item[0] }}.git
    version={{ item[1] }}
    dest=/home/ansible/git/work_tree/{{ item[0] }}
    git_dir=/home/ansible/git/git_dir/{{ item[0] }}.git
  with_nested:
    - ['joyent/http-parser', 'ShareKit/ShareKit']
    - ['v2.3', 'HEAD', 'v2.0']
```