patch: add close() to abstractbackend
patchbackend() seems to call it on an arbitrary backend, so it seems
to be part of the API. Since all subclasses do something in their
close() methods, I decided to let this one raise an exception rather
than just pass.
{header}
<title>Mercurial repository not found</title>
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<body>
<h2>Mercurial repository not found</h2>
The specified repository "{repo|escape}" is unknown, sorry.
Please go back to the <a href="{url|urlescape}">main repository list page</a>.
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