DestroyStackJob: also delete commits

Also use `dependent: :delete` rather than destroy for models
that don't have an `after_destroy` callback.

app/jobs/shipit/destroy_stack_job.rb

@@ -32,6 +32,9 @@ def perform(stack)
 
       delete(Shipit::OutputChunk.joins(:task).where(task: { stack_id: stack.id }))
       delete(Shipit::Task.where(stack_id: stack.id))
+
+      delete(Shipit::Commit.where(stack_id: stack.id))
+
       stack.destroy!
     end
 

app/models/shipit/stack.rb

@@ -38,7 +38,7 @@ def blank?
     has_many :github_hooks, dependent: :destroy, class_name: 'Shipit::GithubHook::Repo'
     has_many :hooks, dependent: :destroy
     has_many :api_clients, dependent: :destroy
-    has_one :continuous_delivery_schedule
+    has_one :continuous_delivery_schedule, dependent: :destroy
     belongs_to :lock_author, class_name: :User, optional: true
     belongs_to :repository
     validates_associated :repository