utxoviewpoint.commit() apparently doesn't always mark entries as unmodified

[cjdelisle]

From reading the code here:

btcd/blockchain/utxoviewpoint.go

Line 462 in ed77733

entry.packedFlags ^= tfModified

I expect that this line should say:

entry.packedFlags &^= tfModified

In order to be sure that the tfModified flag is cleared. As it stands (if I understand correctly), the flag is inverted, so if ever it was unmodified before commit, it will be flagged as modified after.