Merge(backport) r6130 from trunk (#8777).
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.
Authored by: tmaruyama 2011-07-11
Parent: [r6243]
Child: [r6245]