Merge branch 'rebase-use-shared' into 'master'

Use the 'shared' directory for MR rebasing

See merge request !62

app/models/merge_request.rb

@@ -539,7 +539,7 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def rebase_dir_path
-    Rails.root.join('tmp', 'rebase', source_project.id.to_s, id.to_s).to_s
+    File.join(Gitlab.config.shared.path, 'tmp/rebase', source_project.id.to_s, id.to_s).to_s
   end
 
   def rebase_in_progress?