dispatch: handle late KeyboardInterrupt occurred in run()

User can press Ctrl+C while flushing streams in dispatch.run(). In such case,
I think exiting with 255 is better than printing Python traceback and exiting
with 1.

mercurial/dispatch.py

@@ -140,8 +140,10 b' def run():'
                 status = -1
 
         _silencestdio()
-    finally:
-        pass
+    except KeyboardInterrupt:
+        # Catch early/late KeyboardInterrupt as last ditch. Here nothing will
+        # be printed to console to avoid another IOError/KeyboardInterrupt.
+        status = -1
     sys.exit(status & 255)