dispatch: flush ui before returning from dispatch

A chg client may exit after received the result from runcommand. It is
necessary to do a flush to make sure the warning message is printed out
and the process waiting for the chg client will actually see the output.

This helps chg to pass test-alias.t.

mercurial/dispatch.py

@@ -129,6 +129,7 b' def dispatch(req):'
         ret = -1
     finally:
         duration = time.time() - starttime
+        req.ui.flush()
         req.ui.log("commandfinish", "%s exited %s after %0.2f seconds\n",
                    msg, ret or 0, duration)
     return ret