tests/test-issue5979.t

procutil: back out 8403cc54bc83 (make ....procutil.stderr unbuffered)

Changeset 8403cc54bc83 introduced code that opens a second file object
referring to the stderr file descriptor. This broke tests on Windows. The
reason is that on Windows, sys.stderr is buffered and procutil.stderr closed
the file descriptor when it got garbage collected before sys.stderr had the
chance to flush buffered data.

`procutil.stdout` had the same problem for a long time, but we didn’t realize,
as in CI test runs, stdout is not a TTY and in this case no second file object
is opened.

  $ hg init r1
  $ cd r1
  $ hg ci --config ui.allowemptycommit=true -m c0
  $ hg ci --config ui.allowemptycommit=true -m c1
  $ hg ci --config ui.allowemptycommit=true -m c2
  $ hg co -q 0
  $ hg ci --config ui.allowemptycommit=true -m c3
  created new head
  $ hg co -q 3
  $ hg merge --quiet
  $ hg ci --config ui.allowemptycommit=true -m c4

  $ hg log -G -T'{desc}'
  @    c4
  |\
  | o  c3
  | |
  o |  c2
  | |
  o |  c1
  |/
  o  c0
  

  >>> from mercurial import hg
  >>> from mercurial import ui as uimod
  >>> repo = hg.repository(uimod.ui())
  >>> for anc in repo.changelog.ancestors([4], inclusive=True):
  ...   print(anc)
  4
  3
  2
  1
  0