Merge branch '20621-avoid-lame-conflicts-in-ability-allowed' into 'master'

Move abilities by subject class to a dedicated method

## What does this MR do?

Twin of gitlab-org/gitlab-ee!622.

It moves the `case` that returns abilities depending on the subject class to a dedicated method.

This should avoid lame conflicts when CE to EE since the EE version assign the result of the `case` to a variable.

## What are the relevant issue numbers?

Closes #20621.

See merge request !5668

app/models/ability.rb

@@ -6,6 +6,10 @@ class Ability
       return [] unless user.is_a?(User)
       return [] if user.blocked?
 
+      abilities_by_subject_class(user: user, subject: subject)
+    end
+
+    def abilities_by_subject_class(user:, subject:)
       case subject
       when CommitStatus then commit_status_abilities(user, subject)
       when Project then project_abilities(user, subject)