Fix FactorCosetAction(G, G) fo perm. groups (#5903)

Catch the case of an action with trivial image in
`DoFactorCosetActionPerm`,
and delegate to `DoFactorCosetAction` in this case.

lib/csetperm.gi

@@ -883,10 +883,12 @@ local G,u,op,h,N,rt,ac,actions,hom,i,q;
     u:=G;
     Error("only trivial operation ?  I Set u:=G;");
   fi;
+  if IsSubset(u, G) then
+    return DoFactorCosetAction(G, u, G);
+  fi;
   if N=false then
     N:=Core(G,u);
   fi;
-
   ac:=ActionRefinedSeries(G,u);
   actions:=ac[2];
   ac:=ac[1];

lib/factgrp.gd

@@ -167,6 +167,8 @@ DeclareAttribute("NaturalHomomorphismsPool",IsGroup,
 ##
 DeclareOperation( "FactorCosetAction", [IsGroup,IsGroup] );
 
+DeclareGlobalFunction( "DoFactorCosetAction" );
+
 #############################################################################
 ##
 #F  ImproveActionDegreeByBlocks( <G>, <N> , <hom> [,forceblocks] )

lib/factgrp.gi

@@ -475,7 +475,7 @@ end);
 #F  FactorCosetAction( <G>, <U>, [<N>] )  operation on the right cosets Ug
 ##                                        with possibility to indicate kernel
 ##
-BindGlobal("DoFactorCosetAction",function(arg)
+InstallGlobalFunction("DoFactorCosetAction",function(arg)
 local G,u,op,h,N,rt;
   G:=arg[1];
   u:=arg[2];

tst/teststandard/permgrp.tst

@@ -155,6 +155,8 @@ gap> u:=Normalizer(g,SylowSubgroup(g,3));;
 gap> act:=FactorCosetAction(g,u);;
 gap> NrMovedPoints(Range(act));
 70840
+gap> NrMovedPoints(Range(FactorCosetAction(g,g)));
+0
 
 #
 gap> STOP_TEST("permgrp.tst");