	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 by default 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

