FIX: Fp homomorphism test if given on permuted standard generators

This fixes gap-system#3100.

lib/ghomfp.gi

@@ -62,7 +62,8 @@ local s,sg,o,gi;
   o:=One(Range(hom));
   # take the images corresponding to the free gens in case of reordering or
   # duplicates
-  gi:=MappingGeneratorsImages(hom)[2]{hom!.genpositions};
+  gi:=MappingGeneratorsImages(hom)[2]{ListPerm(PermList(hom!.genpositions)^-1,
+    Length(hom!.genpositions))};
   return ForAll(RelatorsOfFpGroup(s),i->MappedWord(i,sg,gi)=o);
 end);
 
@@ -85,7 +86,8 @@ local s, bas, sg, o, gi, l, p, rel, start, i;
   o:=One(Range(hom));
   # take the images corresponding to the free gens in case of reordering or
   # duplicates
-  gi:=MappingGeneratorsImages(hom)[2]{hom!.genpositions};
+  gi:=MappingGeneratorsImages(hom)[2]{ListPerm(PermList(hom!.genpositions)^-1,
+    Length(hom!.genpositions))};
   for rel in RelatorsOfFpGroup(s) do
     l:=LetterRepAssocWord(rel);
     for start in bas do

tst/testbugfix/2018-12-11-fphomgenmix.tst

@@ -0,0 +1,15 @@
+# test for fpgroup homs on mixed generators (fixing #3100)
+gap> f := FreeGroup(18);;
+gap> q := f / [ f.1*f.3*f.7, f.2*f.6*f.8, f.4*f.18*f.14, f.5*f.13*f.9, f.10*f.12*f.16,
+> f.11*f.15*f.17, f.1*f.6, f.2*f.7, f.3*f.18, f.4*f.13, f.5*f.8, f.9*f.12, f.10*f.15, f.11*f.16,
+> f.14*f.17 ];;
+gap> src := [ q.3^-1*q.2^-1, q.2, q.3, q.4, q.5, q.2*q.3, q.7, q.8, q.9, q.10, q.11, q.12, q.13,
+> q.14, q.15, q.16, q.17, q.18 ];;
+gap> dst := [ q.16, q.10, q.12, q.13, q.14, q.11, q.15, q.17, q.18, q.1, q.2, q.3, q.4, q.5, q.6,
+> q.7, q.8, q.9 ];;
+gap> hom := GroupHomomorphismByImages(q,q,src,dst);;
+gap> inv := GroupHomomorphismByImages(q,q,dst,src);;
+gap> IsMapping(hom);
+true
+gap> IsMapping(inv);
+true