Merge branch 'prefer-ref' into 'master'

Prefer ref rather than id because id is shadowing database id

## What does this MR do?

Just a local variable renaming.

## Why was this MR needed?

Prefer ref rather than id because id is shadowing database id.

See merge request !5134

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)

app/models/repository.rb

@@ -78,9 +78,9 @@ class Repository
     end
   end
 
-  def commit(id = 'HEAD')
+  def commit(ref = 'HEAD')
     return nil unless exists?
-    commit = Gitlab::Git::Commit.find(raw_repository, id)
+    commit = Gitlab::Git::Commit.find(raw_repository, ref)
     commit = ::Commit.new(commit, @project) if commit
     commit
   rescue Rugged::OdbError