tests/test-merge5.t

ui: flush stderr after printing a non-chained exception for Windows

There were consistent test failures in test-bad-extension.t, because Windows
buffers stderr when redirected to a file (per the comment in ui.write_err()).
That resulted in failures like this:

  --- c:/Users/Matt/Projects/hg/tests/test-bad-extension.t
  +++ c:/Users/Matt/Projects/hg/tests/test-bad-extension.t.err
  @@ -23,11 +23,11 @@
     Traceback (most recent call last):
     Exception: bit bucket overflow
     *** failed to import extension badext2: No module named badext2
  -  Traceback (most recent call last):
  -  ImportError: No module named badext2
     hg help [-ec] [TOPIC]

     show help for a given topic or a help overview
  +  Traceback (most recent call last):
  +  ImportError: No module named badext2

   show traceback for ImportError of hgext.name if debug is set
   (note that --debug option isn't applied yet when loading extensions)

Instead of inserting another flush immediately after the print, to go along with
the one recently added prior to the print (see 3ff4b07412ad), funnel the output
through ui.write_err().  The flush prior to printing the traceback only mentions
that stdout needs to be flushed, and only stderr needs to be flushed after
printing the traceback.  ui.write_err() does both for us without needing to
redocument the quirky Windows behavior.  It will also clear any progress bar.

  $ hg init
  $ echo This is file a1 > a
  $ echo This is file b1 > b
  $ hg add a b
  $ hg commit -m "commit #0"
  $ echo This is file b22 > b
  $ hg commit -m "comment #1"
  $ hg update 0
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ rm b
  $ hg commit -A -m "comment #2"
  removing b
  created new head
  $ hg update 1
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ hg update
  abort: not a linear update
  (merge or update --check to force update)
  [255]
  $ rm b
  $ hg update -c
  abort: uncommitted changes
  [255]
  $ hg revert b
  $ hg update -c
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
  $ mv a c

Should abort:

  $ hg update 1
  abort: uncommitted changes
  (commit or update --clean to discard changes)
  [255]
  $ mv c a

Should succeed:

  $ hg update 1
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved