util: fix human-readable printing of negative byte counts
Apply the same human-readable printing rules to negative byte counts as to
positive ones. Fixes output of debugupgraderepo.
{header}
<title>Mercurial repository not found</title>
</head>
<body>
<h2>Mercurial repository not found</h2>
The specified repository "{repo|escape}" is unknown, sorry.
Please go back to the <a href="{url|urlescape}">main repository list page</a>.
{footer}