run merge program in repo root.
before, merge program was run in user current directory. Well defined
location is better for scripts. no more need to pass HG_ROOT to merge
program now.
<item>
<title>#desc|strip|firstline|strip|escape#</title>
<link>#url#?cs=#node|short#</link>
<description><![CDATA[#desc|strip|escape|addbreaks#]]></description>
<author>#author|obfuscate#</author>
<pubDate>#date|rfc822date#</pubDate>
</item>