refresh: Support --diff with --edit

[abhinav]

Add a --diff flag to stg refresh that, when used with --edit, is
equivalent to stg edit --diff.

Resolves #98

[jpgrayson]

Want to say thanks for this PR, @abhinav, and sorry I have not yet had time to look at it. I will try to get to it soon.

[jpgrayson]

Merged. I appreciate this PR, @abhinav.

That said, I have mixed feelings about blessing refresh --edit --diff with the same behavior as edit --diff. I agree that viewing the diff inline while editing commit messages is very helpful. BUT, on the other hand, editing diffs along with the commit message is fragile and I consider it an anti-feature of StGit. I have been considering removing the inline diff editing capability. I'm interested in whether anyone relies on this feature in their workflow; and if so, why?

[abhinav]

Thanks, @jpgrayson. Yeah, diff editing was a surprising feature. I use this flag to view, not change. Removal of editing makes sense to me if nobody relies on it heavily.