``pt-online-schema-change`` allowed the name of a constraint to exceed 64 characters when ``--alter-foreign-keys-method=rebuild_constraints`` was used.

The maxium possible length of a constraint name is 64. When --alter-foreign-keys-method=rebuild_constraints used, pt-online-schema-change just adds `_`-character following the below logic:

         if ($fk =~ /^__/) {
           ($new_fk = $fk) =~ s/^__//;
         } else {
           $new_fk = '_'.$fk;  
         }