diff -r a1259e502bdf -r ad56204f733e mercurial/hook.py
--- a/mercurial/hook.py	Thu Oct 20 22:39:59 2016 +0900
+++ b/mercurial/hook.py	Tue Nov 08 22:22:22 2016 +0900
@@ -258,6 +258,7 @@
             sys.stderr.flush()
     finally:
         if _redirect and oldstdout >= 0:
+            sys.__stdout__.flush()  # write hook output to stderr fd
             os.dup2(oldstdout, stdoutno)
             os.close(oldstdout)