Only create refs for new deployments

This patch makes sure GitLab does not save the refs to the filesystem
each time the deployment is updated. This will save some IO although I
expect the impact to be minimal.

app/models/deployment.rb

@@ -11,7 +11,7 @@ class Deployment < ActiveRecord::Base
   delegate :name, to: :environment, prefix: true
-  after_save :create_ref
+  after_create :create_ref
   def commit
     project.commit(sha)