Only set mode of new patch if the target file was removed before.
If the file is writable by the user, but owned by a different user, the
chmod will otherwise fail with "Operation not permitted".
Additionally make very sure that the file is only written if either the number
of links is <= 1 or the file was successfully removed.
Maybe this minimal COW code should be replaced by something from util.
Content-type: text/html; charset={encoding}
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<link rel="icon" href="#staticurl#hgicon.png" type="image/png">
<meta name="robots" content="index, nofollow" />
<link rel="stylesheet" href="#staticurl#style.css" type="text/css" />