Behaviour of DigraphRemoveEdge/DigraphRemoveEdges is inconsistent #260 #498

Job	Run time
--enable-debug --enable-compile-warnings --without-intrinsics	4m 6s
--with-external-bliss --with-external-planarity	8m 52s
--enable-debug --enable-compile-warnings	4m 11s
--with-external-planarity	7m 16s
--with-external-bliss	7m 5s
--enable-compile-warnings --without-intrinsics	4m 0s
--enable-compile-warnings	4m 36s
--enable-debug --without-intrinsics	4m 15s
--enable-debug	4m 11s
--without-intrinsics	4m 3s
	52m 35s

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