Add get_diff method to changesets

[lukaszb]

Synopsis:

def get_diff(self, other)

It should return diff as a unicode. other parameter should be optional. Diff format should be git (requires -g switch to be turned for hg command).

I believe it should be assumed as low level method and simply return strings. We can then create another method that would parse diff and return more structured object. @marcinkuzminski - feedback please :)

Eventually, that method should also allow paths parameter to be passed.

[marcinkuzminski]

nice, just do we implement this as a changeset method or a filenode method ? And again if for example it's a changeset method should it return iterator that returns on string stream or maybe we split this somehow ?