hook: ensure stderr is flushed when an exception is raised, for test stability

Windows has had issues with output order in test-ssh-proto-unbundle.t[1] since
it was created a few weeks ago.  Each of the problems occurred when an exception
was thrown out of the hook.

Now the only thing blocking D2720 is the fact that the "abort: ..." lines on
stderr are totally AWOL.  I have no idea where there are.

[1] https://buildbot.mercurial-scm.org/builders/Win7%20x86_64%20hg%20tests/builds/541/steps/run-tests.py%20%28python%202.7.13%29/logs/stdio

mercurial/hook.py

@@ -265,12 +265,12 b' def runhooks(ui, repo, htype, hooks, thr'
                 raised = False
 
             res[hname] = r, raised
+    finally:
+        # 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.
+        util.stderr.flush()
 
-            # 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.
-            util.stderr.flush()
-    finally:
         if _redirect and oldstdout >= 0:
             util.stdout.flush()  # write hook output to stderr fd
             os.dup2(oldstdout, stdoutno)