scm: return nil at model default_branch and override at git model (#8458, #6713).

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/trunk@6130 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/models/repository.rb

@@ -119,7 +119,7 @@ class Repository < ActiveRecord::Base
   end
 
   def default_branch
-    scm.default_branch
+    nil
   end
 
   def properties(path, identifier=nil)

app/models/repository/git.rb

@@ -75,6 +75,10 @@ class Repository::Git < Repository
     scm.tags
   end
 
+  def default_branch
+    scm.default_branch
+  end
+
   def find_changeset_by_name(name)
     return nil if name.nil? || name.empty?
     e = changesets.find(:first, :conditions => ['revision = ?', name.to_s])