I would not expect a paragraph that starts with "modern" end with
"Sourceforge". They added some half-harted support for Mercurial,
Bazaar, Git some years ago, and people rightly complained that it was
"too late and too little".
There are several reasons to suggest sf. One is the provision of trac.
I'm very keen on having a good issue tracker and haven't been
impressed by what I've seen in Google code or Github when I last looked.
This is way more important to me than support for multiple forked
repositories managed in a social way --- although I agree that doing
this for proofs would be pretty interesting.
I've heard bitbucket.org is nice for Mercurial although I'm not sure if
it's tracker is/will become Jira (Atlassian's well respected commercial
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