Implement gfm_reference directly in Mentionable

Except for Note, which still overrides it.

app/models/commit.rb

@@ -140,11 +140,6 @@ class Commit
     Gitlab::ClosingIssueExtractor.new(project, current_user).closed_by_message(safe_message)
   end
 
-  # Mentionable override.
-  def gfm_reference
-    "commit #{to_reference}"
-  end
-
   def author
     User.find_for_commit(author_email, author_name)
   end

app/models/concerns/mentionable.rb

@@ -20,10 +20,15 @@ module Mentionable
     end
   end
 
-  # Generate a GFM back-reference that will construct a link back to this Mentionable when rendered. Must
-  # be overridden if this model object can be referenced directly by GFM notation.
+  # Returns the text used as the body of a Note when this object is referenced
+  #
+  # By default this will be the class name and the result of calling
+  # `to_reference` on the object.
   def gfm_reference
-    raise NotImplementedError.new("#{self.class} does not implement #gfm_reference")
+    # Convert "MergeRequest" to "merge request"
+    friendly_name = self.class.to_s.underscore.humanize.downcase
+
+    "#{friendly_name} #{to_reference}"
   end
 
   # Construct a String that contains possible GFM references.

app/models/issue.rb

@@ -68,12 +68,6 @@ class Issue < ActiveRecord::Base
     reference
   end
 
-  # Mentionable overrides.
-
-  def gfm_reference
-    "issue #{to_reference}"
-  end
-
   # Reset issue events cache
   #
   # Since we do cache @event we need to reset cache in special cases:

app/models/merge_request.rb

@@ -304,11 +304,6 @@ class MergeRequest < ActiveRecord::Base
     end
   end
 
-  # Mentionable override.
-  def gfm_reference
-    "merge request #{to_reference}"
-  end
-
   def target_project_path
     if target_project
       target_project.path_with_namespace