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