util: copyfile: remove dest before copying

This prevents spurious problems writing to locked files on Windows.

mercurial/util.py

@@ -482,11 +482,9 b' def checksignature(func):'
 
 def copyfile(src, dest):
     "copy a file, preserving mode and atime/mtime"
+    if os.path.lexists(dest):
+        unlink(dest)
     if os.path.islink(src):
-        try:
-            os.unlink(dest)
-        except OSError:
-            pass
         os.symlink(os.readlink(src), dest)
     else:
         try: