fix(events): Change conditions for disabling listeners (fixes #373)

Problem:    Any truthy value returned from an event listener will signal
            the emitter to disable the listener.
Solution:   Only disable the listeners when the returned value is both
            truthy, and of type boolean.

Author: sindrets

lua/diffview/events.lua

@@ -169,10 +169,11 @@ local function filter_call(listeners, event, args)
   for i = 1, #listeners do
     local cur = listeners[i]
     local ret = cur.call(event, args)
+    local discard = (type(ret) == "boolean" and ret)
+        or cur.type == "once"
+        or cur.type == "any_once"
 
-    if not (cur.type == "once" or cur.type == "any_once") and not ret then
-      result[#result + 1] = cur
-    end
+    if not discard then result[#result + 1] = cur end
 
     if not event.propagate then
       for j = i + 1, #listeners do result[j] = listeners[j] end