[PATCH STABLE] cmdutil: fix makefileobj not to clobber default modemap dict

[Yuya Nishihara]

The original bug report is
https://bitbucket.org/tortoisehg/thg/issue/3420/

# HG changeset patch
# User Yuya Nishihara <yuya at tcha.org>
# Date 1382452738 -32400
#      Tue Oct 22 23:38:58 2013 +0900
# Branch stable
# Node ID cfd20b1285b2f5014ba5e87a789436eaf8fdfd9f
# Parent  2c886dedd9021598b6290d95ea0f068731ea4e2b
cmdutil: fix makefileobj not to clobber default modemap dict

Problem occurs if "hg cat -o" is invoked more than once in the same process.
The output of "hg cat" will be appended because of modemap[fn] = 'ab'.

diff --git a/mercurial/cmdutil.py b/mercurial/cmdutil.py
--- a/mercurial/cmdutil.py
+++ b/mercurial/cmdutil.py
@@ -170,7 +170,7 @@ def makefilename(repo, pat, node, desc=N
                          inst.args[0])
 
 def makefileobj(repo, pat, node=None, desc=None, total=None,
-                seqno=None, revwidth=None, mode='wb', modemap={},
+                seqno=None, revwidth=None, mode='wb', modemap=None,
                 pathname=None):
 
     writable = mode not in ('r', 'rb')
@@ -198,9 +198,10 @@ def makefileobj(repo, pat, node=None, de
     if util.safehasattr(pat, 'read') and 'r' in mode:
         return pat
     fn = makefilename(repo, pat, node, desc, total, seqno, revwidth, pathname)
-    mode = modemap.get(fn, mode)
-    if mode == 'wb':
-        modemap[fn] = 'ab'
+    if modemap is not None:
+        mode = modemap.get(fn, mode)
+        if mode == 'wb':
+            modemap[fn] = 'ab'
     return open(fn, mode)
 
 def openrevlog(repo, cmd, file_, opts):