Fixed the --gain option for balance reports

doc/ledger.1

@@ -86,6 +86,7 @@ and causes an immediate reloading of all journal files in the session.
 .Bl -tag -width -indent
 .It Fl \-abbrev-len Ar INT
 .It Fl \-account Ar STR
+.It Fl \-account-amount Ar EXPR
 .It Fl \-account-width Ar INT
 .It Fl \-actual Pq Fl L
 .It Fl \-add-budget

src/account.cc

@@ -326,6 +326,7 @@ void account_t::calculate_sums(expr_t& amount_expr)
   value_t amount(amount_expr.calc(bound_scope));
 
   if (! amount.is_null()) {
+    DEBUG("account.sums", "Added " << amount << " to " << fullname());
     add_or_set_value(xd.total, amount);
     xd.total_count += xd.count;
   } else {

src/report.h

@@ -396,6 +396,7 @@ public:
 
   OPTION_(report_t, gain, DO() { // -G
       parent->HANDLER(revalued).on_only();
+      parent->HANDLER(account_amount_).set_expr("amount | (0, 0)");
       parent->HANDLER(amount_).set_expr("(amount, cost)");
       // Since we are displaying the amounts of revalued postings, they
       // will end up being composite totals, and hence a pair of pairs.