	Update the current or specified 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.

	After the update the current branch is the specified one.

	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

