Don’t always reset the completion on command execution

e.g. if there is nothing to reset

src/tabs/basetabs.py

@@ -290,6 +290,7 @@ class Tab(object):
             else:
                 arg = xhtml.clean_text_simple(arg)
             if func:
+                if hasattr(self.input, "reset_completion"):
                     self.input.reset_completion()
                 func(arg)
             return True