color: document the possibility to colourise tabs in diffs This is a no-op change that simply suggests that tabs can be colourised, but defaults to no colour effects for tabs. This complements cset c343557a8442. Like the similar effects for phases from f8e2aebbb24c, we set the default effects to '' instead of None, so that this is a true noop change. Otherwise, the diff.tab effect would override (i.e. neutralise) the effect of the surrounding label.