cat: default to working dir parent instead of tip
This introduces a defaultrev function that chooses
the working dir parent if a revision isn't specified,
and uses it in several places.
default = 'changelog'
header = header-rss.tmpl
changelog = changelog-rss.tmpl
changelogentry = changelogentry-rss.tmpl
filelog = filelog-rss.tmpl
filelogentry = filelogentry-rss.tmpl
tags = tags-rss.tmpl
tagentry = tagentry-rss.tmpl