util: move copymode into posix.py and windows.py reducing it to a NOP on Windows. This eliminates a pointless stat call on Windows and reduces the risk of interferring with other processes (e.g. AV-scanners, file change watchers). See also http://mercurial.selenic.com/wiki/UnlinkingFilesOnWindows, item 2d

--- a/mercurial/windows.py	Tue Aug 02 12:29:48 2011 +0200
+++ b/mercurial/windows.py	Tue Aug 02 13:18:56 2011 +0200
@@ -105,6 +105,9 @@
 def setflags(f, l, x):
     pass
 
+def copymode(src, dst, mode=None):
+    pass
+
 def checkexec(path):
     return False