commit_description helper method

same commit description in commit#index as in commit#show

I have white space at description beginning but this isn't
![screenshot at 13 13-19-20](https://cloud.githubusercontent.com/assets/1488195/3905225/d6486d04-22e4-11e4-9356-5017ef3f151b.png)
so fix that
![screenshot at 13 13-18-41](https://cloud.githubusercontent.com/assets/1488195/3905235/ed431270-22e4-11e4-9f22-0698f9f05d6d.png)

at commits#show commit description looks another rather than commit#show . Look at html tags and refs.
![screenshot at 13 15-47-51](https://cloud.githubusercontent.com/assets/1488195/3905264/4fa0621a-22e5-11e4-869e-002802a03bc5.png)
also fix that
![Uploading Screenshot at авг. 13 15-49-02.png . . .]()


Unused method argument - `commit`. If it's necessary, use `_` or `_commit` as an argument name to indicate that it won't be used. You can also write as `commit_description(*)` if you want the method to accept any arguments but don't care about them.