diff -r 1439bacf0ec7 -r 2abbf4750915 mercurial/hook.py
--- a/mercurial/hook.py	Fri Apr 10 22:37:40 2015 -0400
+++ b/mercurial/hook.py	Fri Apr 10 22:30:19 2015 -0400
@@ -200,6 +200,11 @@
                 r = _pythonhook(ui, repo, name, hname, hookfn, args, throw) or r
             else:
                 r = _exthook(ui, repo, hname, cmd, args, throw) or r
+
+            # The stderr is fully buffered on Windows when connected to a pipe.
+            # A forcible flush is required to make small stderr data in the
+            # remote side available to the client immediately.
+            sys.stderr.flush()
     finally:
         if _redirect and oldstdout >= 0:
             os.dup2(oldstdout, stdoutno)