Add searching for named branches
named branching is when you identify a head by a tag on an earlier revision.
This patch adds repo.branchlookup for searching through the tree to
find branch tags for heads.
hg update -b tag is added to checkout based on branch tags
hg heads -b is added to print the tag associated with each head
<item>
<title>#desc|firstline|escape#</title>
<link>#url#?cmd=changeset;node=#node#</link>
<description><![CDATA[#desc|escape|addbreaks#]]></description>
<author>#author|obfuscate#</author>
<pubDate>#date|rfc822date#</pubDate>
</item>