osutil: eliminate alloca call
- make full_path a PATH_MAX + epsilon local buffer
- use strncpy and strncat
<entry>
<title>#tag|escape#</title>
<link rel="alternate" href="{urlbase}{url}rev/{node}"/>
<id>http://www.selenic.com/mercurial/#tag-{node}</id>
<updated>#date|rfc3339date#</updated>
<published>#date|rfc3339date#</published>
<content type="text">#tag|strip|escape#</content>
</entry>