dispatch: adjust ui.flush() timing to stabilize test-blackbox.t

Without this change, dispatch.dispatch() could return before flushing all
stdio data. This means chg stdio would print data after receiving the result
code.

--- tests/test-blackbox.t
+++ tests/test-blackbox.t.err
@@ -354,13 +354,13 @@
   > EOF
   $ hg log --debug
   removing $TESTTMP/gone/.hg
-  warning: cannot write to blackbox.log: $ENOENT$ (no-windows !)
   warning: cannot write to blackbox.log: $TESTTMP/gone/.hg/blackbox.log: $ENOTDIR$ (windows !)
   $ cd ..

 blackbox should disable itself if track is empty

   $ hg --config blackbox.track= init nothing_tracked
+  warning: cannot write to blackbox.log: $ENOENT$
   $ cd nothing_tracked
   $ cat >> .hg/hgrc << EOF
   > [blackbox]