determinize: do not copy the "incomplete" property

Mentioned in issue #298.

* spot/twaalgos/determinize.cc: Do not copy prop_complete of
the input if it is false.
* NEWS: Mention the bug.

NEWS

@@ -9,6 +9,10 @@ New in spot 2.9.5.dev (not yet released)
     performing an (unnecessary) second iteration even when the first
     one failed to reduce the automaton.  (Issue #442)
 
+  - When passed an incomplete automaton as input, tgba_determinize()
+    would sometimes produce a complete automaton but incorrectly mark
+    it as incomplete.
+
 New in spot 2.9.5 (2020-11-19)
 
   Bugs fixed:

spot/twaalgos/determinize.cc

@@ -926,9 +926,12 @@ namespace spot
                    { false, // state based
                        false, // inherently_weak
                        false, false, // deterministic
-                       true, // complete
+                       false, // complete
                        true // stutter inv
                        });
+    // completeness can only be improved.
+    if (aut->prop_complete().is_true())
+      res->prop_complete(true);
 
     // Given a safra_state get its associated state in output automata.
     // Required to create new edges from 2 safra-state