bundle2: add a ``bundle20.nbparts`` property
This property can be used to know how much parts have been added to the bundle2.
This will be useful to check if any part have been generated for a push.
<entry>
<title>{branch|escape}</title>
<link rel="alternate" href="{urlbase}{url|urlescape}rev/{node|short}"/>
<id>{urlbase}{url|urlescape}#branch-{node}</id>
<updated>{date|rfc3339date}</updated>
<published>{date|rfc3339date}</published>
<content type="text"><![CDATA[{branch|strip|escape|addbreaks}]]></content>
</entry>