Add a verbose flag to pull-request command

`git commit` has a verbose flag that outputs the diff of the changes you're about to commit. Here, I'm adding the flag that does the same thing: output the diff of the changes that you're going to submit a pull request for.

The diff for `git commit -v` is not in comments and I think that's done intentionally for text editors to color code the message. So, I did a search for how `git` knows not to include the diff in the commit message and found [this](http://www.spinics.net/lists/git/msg221525.html). I've never coded in C but it seems like it searches for `diff --git ` text and ignores everything below it. That's how I implemented it in pull request.

However, that patch also shows the issues with the implementation and how they want to change it to a 'special scissor separator line'. We can change this implementation to use a 'special line' as well but it's not the current `git` behavior so I didn't do that. 

Let me know what you think!