filemerge: add internal:dump
This create foo.{local,other,base} files for people to manually merge
files while littering their working directory.
<item>
<title>{desc|strip|firstline|strip|escape}</title>
<link>{urlbase}{url}log{{node|short}}/{file|urlescape}</link>
<description><![CDATA[{desc|strip|escape|addbreaks|nonempty}]]></description>
<author>{author|obfuscate}</author>
<pubDate>{date|rfc822date}</pubDate>
</item>