localrepo: add some basic comment for block in __getitem__
There are different early processing before getting to the core of the function.
We highlight that fact.
Differential Revision: https://phab.mercurial-scm.org/D7473
<item>
<title>{branch|escape}</title>
<link>{urlbase}{url|urlescape}rev/{node|short}</link>
<description><![CDATA[{branch|strip|escape|addbreaks}]]></description>
<pubDate>{date|rfc822date}</pubDate>
</item>