Vault.sellUSDG() incorrect feeBasisPoints

[frontier159]

gm,

The fee calc when selling GLP (the vault.sellUSDG() function) may have a bug.

When selling GLP, the fee for the token to sell into is calculated here:
https://github.com/gmx-io/gmx-contracts/blob/master/contracts/core/Vault.sol#L508

This underlying fee calc depends on vault.usdgAmounts(usg) for the initialAmount:
https://github.com/gmx-io/gmx-contracts/blob/master/contracts/core/VaultUtils.sol#L146

However this usdgAmount has already been updated (decreased) prior to this being called
https://github.com/gmx-io/gmx-contracts/blob/master/contracts/core/Vault.sol#L497

So it looks like the initialAmount var isn't the initial amount, it's already had the usdgDelta applied to it.

Also as far as I can tell, the UI quote is inconsistent with this contract calc - since the usdgAmount being used is the amount prior to the transaction.
https://github.com/gmx-io/gmx-interface/blob/master/src/Helpers.js#L655

Apart from potentially taking more fees than necessary, I guess if the UI quote is different from the actual calc, it may have slippage issues for users.

[frontier159]

To illustrate how to match the UI code with what the contract is actually currently doing:
https://github.com/gmx-io/gmx-interface/blob/master/src/Helpers.js#L579

Using increment=false as the signal for a sell in getFeeBasisPoints()

  const initialAmount = token.usdgAmount;
  if (!increment) {
    // The contract code already decrements the initial amount by the amount to sell.
    // So to have a matching quote, the same logic needs to be added here.
    initialAmount = initialAmount.lte(usdgDelta) ? bigNumberify(0) : initialAmount.sub(usdgDelta);
  }
  let nextAmount = ...

But I feel like this is a bug in the contract code - so raising here.