--- a/mercurial/hook.py	Wed Nov 16 12:53:10 2011 +0100
+++ b/mercurial/hook.py	Wed Nov 16 08:34:36 2011 +0100
@@ -139,6 +139,7 @@
             stderrno = sys.__stderr__.fileno()
             # temporarily redirect stdout to stderr, if possible
             if stdoutno >= 0 and stderrno >= 0:
+                sys.__stdout__.flush()
                 oldstdout = os.dup(stdoutno)
                 os.dup2(stderrno, stdoutno)
         except AttributeError: