osutil: fix potential wrong fd close

According to POSIX closedir [1]:

  If a file descriptor is used to implement type DIR, that file descriptor
  shall be closed.

According to POSIX fdopendir [2]:

  Upon calling closedir() the file descriptor shall be closed.

So we should avoid "close(dfd)" after "closedir(dir)". With threads, there
could be a race where an innocent fd gets closed. But Python GIL seems to
help hiding the issue well.

[1]: http://pubs.opengroup.org/onlinepubs/009695399/functions/closedir.html
[2]: http://pubs.opengroup.org/onlinepubs/9699919799/functions/fdopendir.html

mercurial/osutil.c

@@ -400,6 +400,8 b' error:'
 	Py_XDECREF(stat);
 error_list:
 	closedir(dir);
+	/* closedir also closes its dirfd */
+	goto error_value;
 error_dir:
 #ifdef AT_SYMLINK_NOFOLLOW
 	close(dfd);