progress: determine padding width portably
Differential Revision: https://phab.mercurial-scm.org/D2225
{header}
<id>{urlbase}{url|urlescape}atom-log/tip/{file|escape}</id>
<link rel="self" href="{urlbase}{url|urlescape}atom-log/tip/{file|urlescape}"/>
<title>{repo|escape}: {file|escape} history</title>
{latestentry%feedupdated}
{entries%changelogentry}
</feed>