context: use "vfs.lstat()" to examine target path instead of "os.path.*"

This patch gets stat object of target path by "vfs.lstat()", and
examines stat object to know the type of it. This follows the way in
"workingctx.add()".

This should be cheaper than original implementation invoking
"lexists()", "isfile()" and "islink()".

mercurial/context.py

@@ -1173,10 +1173,14 b' class workingctx(committablectx):'
             wlock.release()
 
     def copy(self, source, dest):
-        p = self._repo.wjoin(dest)
-        if not os.path.lexists(p):
+        try:
+            st = self._repo.wvfs.lstat(dest)
+        except OSError, err:
+            if err.errno != errno.ENOENT:
+                raise
             self._repo.ui.warn(_("%s does not exist!\n") % dest)
-        elif not (os.path.isfile(p) or os.path.islink(p)):
+            return
+        if not (stat.S_ISREG(st.st_mode) or stat.S_ISLNK(st.st_mode)):
             self._repo.ui.warn(_("copy failed: %s is not a file or a "
                                  "symbolic link\n") % dest)
         else: