Homomorphism finding is sometimes unreasonably slow

[james-d-mitchell]

For example,

gap> DigraphHomomorphism(NullDigraph(2), NullDigraph(64));
IdentityTransformation
gap> time;
1565

This seems to be because the homomorphism finding code finds a stabiliser chain for the symmetric group on 64 points, which is unreasonably slow, even though the very first attempt at finding a homomorphism would result in the IdentityTransformation being discovered. Not sure exactly how to fix this.