	Generate a patch from the current or specified topic branch.
	This means that the diff between the topic branch base and head
	(latest commit) is shown, appended to the description found in
	the '.topmsg' file.

	The patch is simply dumped to stdout.  In the future, `tg patch`
	will be able to automatically send the patches by mail or save
	them to files. (TODO)

	Options:
	  -i		base patch generation on index instead of branch
	  -w		base patch generation on working tree instead of branch

