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scm: return nil at model default_branch and override at git model. Redmine Git and Mercurial adapter support *branches*. Mercurial default branch is *dafault*. But, it is popular to show all revisions on the top page for Mercurial GUI. git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/branches/1.2-stable@6244 e93f8b46-1217-0410-a6f0-8f06a7374b81