Add dry-run argument to patch command.

[fuzzball81]

A dry-run option would allow a user to see the actions patch would perform without actually running the operations themselves. If the patches branch contained two new patches so example output might be

Add: 0002-first-new-patch
Add: 0003-second-new-patch

Spec version 1.0.0-1 -> 1.0.0-2
Changelog:
Mon May 15 2017 Your User [email protected]:

• First new patch
• Second new patch

If patches were removed they would be listed as removed patches. Note sure how to handle renumbered once, it would be nice if those showed up as Renumber 000X to 000Y.