	Update the current topic branch wrt. changes in the branches
	it depends on and remote branches.
	This is performed in two phases - first,
	changes within the dependencies are merged to the base,
	then the base is merged into the topic branch.
	The output will guide you in case of conflicts.

	In case your dependencies are not up-to-date, tg update
	will first recurse into them and update these.

	If a remote branch update brings dependencies on branches
	not yet instantiated locally, you can either bring in all
	the new branches from the remote using 'tg remote --populate'
	or only pick out the missing ones using 'tg create -r'
	('tg summary' will point out branches with incomplete
	dependencies by showing an '!' near to them).

	TODO: tg update -a for updating all topic branches

