identify: have consistent output for local repositories
Also, add some extensive testing for it (hardly any in the suite so far).
{header}
<!-- Error -->
<id>{urlbase}{url}</id>
<link rel="self" href="{urlbase}{url}atom-log"/>
<link rel="alternate" href="{urlbase}{url}"/>
<title>Error</title>
<updated>1970-01-01T00:00:00+00:00</updated>
<entry>
<title>Error</title>
<id>http://www.selenic.com/mercurial/#error</id>
<author>
<name>mercurial</name>
</author>
<updated>1970-01-01T00:00:00+00:00</updated>
<content type="text">{error|escape}</content>
</entry>
</feed>