Adding tree aliase

Example:
```
$ git tree
* a9f38f4 (HEAD, origin/master, origin/HEAD, master) Add tree aliase
*   7ca9285 Merge pull request #2 from smmoosavi/patch-1
|\  
| * eb011c6 (origin/patch-1) add comment to .gitconfig
|/  
* 1a0f443 Adding gittify script and configs/aliases git files
* 7993e98 Writing a README in markdown syntax about usages and installation method
* f311065 Adding GPLv3 LICENSE file
```

source: http://stackoverflow.com/questions/1064361/unable-to-show-a-git-tree-in-terminal/5354644#5354644