internal/t8ntool: rework how isMerged is determined

Author: holiman

cmd/evm/internal/t8ntool/transition.go

@@ -252,8 +252,9 @@ func Transition(ctx *cli.Context) error {
 			return NewError(ErrorConfig, errors.New("EIP-1559 config but missing 'currentBaseFee' in env section"))
 		}
 	}
-
-	if env := prestate.Env; chainConfig.IsMerged(big.NewInt(int64(env.Number))) {
+	isMerged := chainConfig.TerminalTotalDifficulty != nil && chainConfig.TerminalTotalDifficulty.BitLen() == 0
+	env := prestate.Env
+	if isMerged {
 		// post-merge:
 		// - random must be supplied
 		// - difficulty must be zero

params/config.go

@@ -477,11 +477,6 @@ func (c *ChainConfig) IsArrowGlacier(num *big.Int) bool {
 	return isForked(c.ArrowGlacierBlock, num)
 }
 
-// IsMerged returns whether num is either equal to the TheMerge fork block or greater.
-func (c *ChainConfig) IsMerged(num *big.Int) bool {
-	return isForked(c.MergeForkBlock, num)
-}
-
 // IsTerminalPoWBlock returns whether the given block is the last block of PoW stage.
 func (c *ChainConfig) IsTerminalPoWBlock(parentTotalDiff *big.Int, totalDiff *big.Int) bool {
 	if c.TerminalTotalDifficulty == nil {