tests/test-ancestor.py.out

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

% lazy ancestor set for [], stoprev = 0, inclusive = False
membership: []
iteration:  []
% lazy ancestor set for [11, 13], stoprev = 0, inclusive = False
membership: [7, 8, 3, 4, 1, 0]
iteration:  [3, 7, 8, 1, 4, 0, 2]
% lazy ancestor set for [1, 3], stoprev = 0, inclusive = False
membership: [1, 0]
iteration:  [0, 1]
% lazy ancestor set for [11, 13], stoprev = 0, inclusive = True
membership: [11, 13, 7, 8, 3, 4, 1, 0]
iteration:  [11, 13, 3, 7, 8, 1, 4, 0, 2]
% lazy ancestor set for [11, 13], stoprev = 6, inclusive = False
membership: [7, 8]
iteration:  [7, 8]
% lazy ancestor set for [11, 13], stoprev = 6, inclusive = True
membership: [11, 13, 7, 8]
iteration:  [11, 13, 7, 8]