Wrap JGit's optional parameters for creating diff patches

[dometto]

RJGit::Porcelain.diff(repo, :patch => true) will create an array of diff entry hashes with a git patch in the :patch key. The method does not yet allow passing in optional formatting parameters that JGit supports:

http://download.eclipse.org/jgit/docs/jgit-2.0.0.201206130900-r/apidocs/org/eclipse/jgit/diff/DiffFormatter.html

(setContext, setNewPrefix, etc.)

There is a pending spec for this in spec/rjgit_spec.rb