IsHamiltonianDigraph attempts to modify immutable blists

[wilfwilson]

As highlighted in a recent handy Travis CRON job, IsHamiltonianDigraph modifies mutable lists. This would mean that the attribute BooleanAdjacencyMatrix would potentially by modified by running IsHamiltonianDigraph. That this didn't previously raise an error was a bug in GAP, which has just been fixed in GAP master branch in gap-system/gap#3392. Now we get the following error:

gap> g := Digraph([[3], [3], [1, 2]]);
<digraph with 3 vertices, 4 edges>
gap> IsHamiltonianDigraph(g);
Error, IntersectBlist: <blist1> must be a mutable boolean list (not a list (boolean,imm)) in
  IntersectBlist( tempblist, adjmatrix[j] );