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>Mercurial Error</title>
</head>
<body>
<h2>Mercurial Error</h2>
<p>
An error occurred while processing your request:
</p>
<p>
{error|escape}
</p>
{footer}