localrepo: improve readability of _findtags(), readtags() (issue548).
- rename many local variables
- add some comments
- refactor call to line.split() (catch ValueError rather
than checking length of return value: one less local variable)
<entry>
<title>{tag|escape}</title>
<link rel="alternate" href="{urlbase}{url}rev/{node}"/>
<id>{urlbase}{url}#tag-{node}</id>
<updated>{date|rfc3339date}</updated>
<published>{date|rfc3339date}</published>
<content type="text">{tag|strip|escape}</content>
</entry>