Inefficiency with `UndirectedSpanningTree` method for a mutable digraph

[wilfwilson]

The method for UndirectedSpanningTree for IsMutableDigraph creates two undirected spanning forests, but it should be able to get by with creating only one:

Digraphs/gap/attr.gi

Lines 2283 to 2293 in 119adb3

	InstallMethod(UndirectedSpanningTree, "for a mutable digraph",
	[IsMutableDigraph],
	function(D)
	if not (DigraphHasAVertex(D)
	and IsStronglyConnectedDigraph(D)
	and IsConnectedDigraph(UndirectedSpanningForest(DigraphMutableCopy(D))))
	then
	return fail;
	fi;
	return UndirectedSpanningForest(D);
	end);

Specifically, an undirected spanning tree is created as part of the big if statement, and then again at the end.

I believe that eliminating this inefficiency would need only a simple refactoring and should be an doable for a newcomer to Digraphs.