ledger-display-balance-at-point: Use buffer if too long

`ledger cleared` can produce output with quite a few lines, especially for
accounts with subaccounts or multiple commodities.  display-message-or-buffer
will show output in a buffer if it is too long to fit in the echo area (before
this commit, only the last few lines of the output would be displayed at all).

ledger-mode.el

@@ -137,7 +137,7 @@ the balance into that."
                         (buffer-substring-no-properties (point-min) (1- (point-max)))
                       (concat account " is empty.")))))
     (when balance
-      (message balance))))
+      (display-message-or-buffer balance))))
 
 (defun ledger-display-ledger-stats ()
   "Display some summary statistics about the current ledger file."