The changelog is available as CHANGES.txt in the SCM repository.
Read-only repository access is provided through
hg clone http://hg.code.sf.net/p/roundup/code roundup-code
The URL for the webinterface works, too, but you will see messages about redirects to the URL shown here.
The read/write access uses your SourceForge.net ssh password or ssh key to authorize your access. (See SF’s site documentation on Mercurial access)
hg clone ssh://USERNAME@hg.code.sf.net/p/roundup/code roundup-code
Of course a roundup developer must have granted you write access first - ask for it on the roundup-devel list.