cmdutil: do not duplicate stdout by makefileobj()

It made output order unpredictable because two separate buffers are flushed
individually. Let's use a thin wrapper that just sends close() to black hole.

mercurial/cmdutil.py

@@ -462,11 +462,6 b' def makefileobj(repo, pat, node=None, de'
             fp = repo.ui.fout
         else:
             fp = repo.ui.fin
-        if util.safehasattr(fp, 'fileno'):
-            return os.fdopen(os.dup(fp.fileno()), mode)
-        else:
-            # if this fp can't be duped properly, return
-            # a dummy object that can be closed
             return _unclosablefile(fp)
     if util.safehasattr(pat, 'write') and writable:
         return pat

tests/test-export.t

@@ -140,6 +140,7 b' Exporting revision -2 to a file:'
 No filename should be printed if stdout is specified explicitly:
 
   $ hg export -v 1 -o -
+  exporting patch:
   # HG changeset patch
   # User test
   # Date 0 0
@@ -154,7 +155,6 b' No filename should be printed if stdout '
   @@ -1,1 +1,2 @@
    foo-0
   +foo-1
-  exporting patch:
 
 Checking if only alphanumeric characters are used in the file name (%m option):