Use plumbing version of git command to retrieve the current working branch

When I had first downloaded this to give it a whirl and tried to list the branches I kept getting.

```
phpenv v0.0.4-dev

Already on 'master'
```
Turns out this was because of the following check to see if the current php-src checkout was @ master was returning false all of the time when it was on master because I am using colored output for git.  

```bash

if ${GIT} branch | grep -q "* master"; then

```

To prevent this we should use the plumbing command ```rev-parse``` instead of the porcelain command ```branch```.

For reference:
- http://stackoverflow.com/questions/1417957/show-just-the-current-branch-in-git

  A discussion on retrieving the current branch for a git repo.

- http://git-scm.com/book/en/Git-Internals-Plumbing-and-Porcelain

  Plumbing/Porcelain

- https://www.kernel.org/pub/software/scm/git/docs/git-rev-parse.html


-- Jonathan