Clarify precedence for template/style in commands.show_changeset():
Display format will be the first non-empty hit of:
1. option 'template'
2. option 'style'
3. [ui] setting 'logtemplate'
4. [ui] setting 'style'
If all of these values are either the unset or the empty string,
regular display via changeset_printer() is done.
<item>
<title>#tag|escape#</title>
<link>#url#?cs=#node|short#</link>
<description><![CDATA[#tag|strip|escape|addbreaks#]]></description>
<pubDate>#date|rfc822date#</pubDate>
</item>