Added special branch '_' in git_pillar to match saltenv

I prefer a setup where states/pillar come from a remote git repository. Something like:

```
## srv/salt/top.sls:
{{env}}:
  - bar

## srv/pillar/top.sls:
{{env}}:
  - bar
```
Such that I can do:
`salt '*' state.highstate test=True saltenv=my_test_branch`

This will effectively translated to:
```
## srv/salt/top.sls:
my_test_branch:
  - bar

## srv/pillar/top.sls:
my_test_branch:
  - bar
```

Corresponding master config:
```
fileserver_backend:
  - git

gitfs_remotes:
    - file:///var/tmp/saltmaster:
        - root: salt

ext_pillar:
  - git: _ file:///var/tmp/saltmaster root=pillar
```