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
{header}
<title>{repo|escape}: tags</title>
<description>{repo|escape} tag history</description>
{entriesnotip%tagentry}
</channel>
</rss>