Add StaticModel role, and add it to Commit model

Instead of doing this:

    link_to(commit.id, project_commit_path(project, id: commit.id))
    Note.create(noteable_id: commit.id, noteable_type: "Commit", ...)

It lets us do this:

    link_to(commit.id, project_commit_path(project, commit))
    Note.create(noteable: commit, ...)

app/models/commit.rb

@@ -1,6 +1,7 @@
 class Commit
   include ActiveModel::Conversion
   include Gitlab::Encode
+  include StaticModel
   extend ActiveModel::Naming
 
   attr_accessor :commit
@@ -22,7 +23,6 @@ class Commit
     :to_patch,
     to: :commit
 
-
   class << self
     def find_or_first(repo, commit_id = nil, root_ref)
       commit = if commit_id
@@ -105,10 +105,6 @@ class Commit
     end
   end
 
-  def persisted?
-    false
-  end
-
   def initialize(raw_commit, head = nil)
     @commit = raw_commit
     @head = head

app/roles/static_model.rb

@@ -0,0 +1,35 @@
+# Provides an ActiveRecord-like interface to a model whose data is not persisted to a database.
+module StaticModel
+  extend ActiveSupport::Concern
+
+  module ClassMethods
+    # Used by ActiveRecord's polymorphic association to set object_id
+    def primary_key
+      'id'
+    end
+
+    # Used by ActiveRecord's polymorphic association to set object_type
+    def base_class
+      self
+    end
+  end
+
+  # Used by AR for fetching attributes
+  #
+  # Pass it along if we respond to it.
+  def [](key)
+    send(key) if respond_to?(key)
+  end
+
+  def to_param
+    id
+  end
+
+  def persisted?
+    false
+  end
+
+  def destroyed?
+    false
+  end
+end