convert: svn: pull up path to file expansion code into separate function.
This is part of the process for deferring path expansion until getchanges.
Copy detection also needs to be moved out of the commit object.
<item>
<title>#desc|strip|firstline|strip|escape#</title>
<link>{urlbase}{url}log{#node|short#}/{file|urlescape}</link>
<description><![CDATA[#desc|strip|escape|addbreaks#]]></description>
<author>#author|obfuscate#</author>
<pubDate>#date|rfc822date#</pubDate>
</item>