Prefer ref rather than id because id is shadowing database id

Some context:
http://doc.gitlab.com/ce/api/repository_files.html#get-file-from-repository
http://doc.gitlab.com/ce/api/repositories.html#list-repository-tree

Slack:
https://gitlab.slack.com/archives/questions/p1467890450002077

app/models/project.rb

@@ -425,8 +425,8 @@ class Project < ActiveRecord::Base
     container_registry_repository.tags.any?
   end
 
-  def commit(id = 'HEAD')
-    repository.commit(id)
+  def commit(ref = 'HEAD')
+    repository.commit(ref)
   end
 
   def merge_base_commit(first_commit_id, second_commit_id)