SCM: Git clone performance

Optimize git clone.

Via:
- http://stackoverflow.com/questions/1778088/how-to-clone-a-single-branch-in-git
- http://stackoverflow.com/questions/6941889/is-git-clone-depth-1-shallow-clone-more-useful-than-it-makes-out
- http://git-scm.com/docs/git-clone


### Test:
Clone https://github.com/phalcon/cphalcon from 1.3.3 branch.

#### 1 Before. 
```
$ git clone https://github.com/phalcon/cphalcon.git
Cloning into 'cphalcon'...
remote: Counting objects: 68626, done.
remote: Total 68626 (delta 0), reused 0 (delta 0)
Receiving objects: 100% (68626/68626), 57.36 MiB | 1.55 MiB/s, done.
Resolving deltas: 100% (52217/52217), done.
$ git checkout 1.3.3
```


#### 2. After.
```
$git clone --depth 1 -b 1.3.3 https://github.com/phalcon/cphalcon.git
Cloning into 'cphalcon'...
remote: Counting objects: 8255, done.
remote: Compressing objects: 100% (4588/4588), done.
remote: Total 8255 (delta 5601), reused 5283 (delta 3383)
Receiving objects: 100% (8255/8255), 9.63 MiB | 1.13 MiB/s, done.
Resolving deltas: 100% (5601/5601), done.
```
( -q=Operate quietly. Progress is not reported to the standard error stream.)


#### Reesults: 57.36 MiB vs 9.63 MiB ;)