Close key binding window when reassigning the same key

This allows for canceling a key reassignment. We cannot use a static key
here since every key could potentially be used in a binding. Instead,
just cancel if we are trying to reassign a key that has already been
used for that action before.

Signed-off-by: Michael Smith <crazedpsyc@mail4us.net>
Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/custom.c

@@ -980,6 +980,12 @@ void custom_keys_config(void)
         } else
           not_recognized = 0;
 
+        /* Is the binding used by this action already? If so, just end the reassignment */
+        if (selrow == keys_get_action(keyval)) {
+          delwin(grabwin);
+          break;
+        }
+
         used = keys_assign_binding(keyval, selrow);
         if (used) {
           enum key action;