Fix TGraph copy constructor and assign operator (#7302)
Fix TGraph copy constructor and assign operator (#7302)

Cloned histogram should not be assign to current directory

olifre/root,olifre/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root,olifre/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,root-mirror/root,olifre/root