[assistant-stock-transaction] input positive capgains for Credit inco…

…me acct

In a long position, a positive capital gain indicates the unit price
has increased from Buy to Sell.

In a short position, a positive capital gain indicates the unit price
has decreased from Short Sell to Cover Buy.

i.e. when inputting capital gains, the UI amount being positive
indicates the income account has a credit value. i.e. the investor has
enjoyed a 'profit' on their transaction.

gnucash/gnome/assistant-stock-transaction.cpp

@@ -292,7 +292,7 @@ static const TxnTypeVec short_types
         FieldMask::ENABLED_DEBIT | FieldMask::ALLOW_ZERO,          // fees_amt
         false,                  // fees_capitalize
         FieldMask::DISABLED,               // dividend_amt
-        FieldMask::ENABLED_DEBIT | FieldMask::ALLOW_ZERO | FieldMask::ALLOW_NEGATIVE,          // capg_amt
+        FieldMask::ENABLED_CREDIT | FieldMask::ALLOW_ZERO | FieldMask::ALLOW_NEGATIVE,          // capg_amt
         // Translators: this is a stock transaction describing cover
         // buying stock, and recording capital gain/loss
         N_("Buy to cover short"),