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.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/branches/1.2-stable@6244 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/models/repository.rb

@@ -117,7 +117,7 @@ class Repository < ActiveRecord::Base
   end
 
   def default_branch
-    scm.default_branch
+    nil
   end
 
   def properties(path, identifier=nil)