Rename MergeRequest#lock! to #lock_mr!

As a warning from the StateMachine gem points out to us, there already
exists a method ActiveRecord#lock!. This change renames the `lock` and
`unlock` events defined on MergeRequest to `lock_mr` and `unlock_mr`.

app/models/merge_request.rb

@@ -62,11 +62,11 @@ class MergeRequest < ActiveRecord::Base
       transition closed: :reopened
     end
 
-    event :lock do
+    event :lock_mr do
       transition [:reopened, :opened] => :locked
     end
 
-    event :unlock do
+    event :unlock_mr do
       transition locked: :reopened
     end
 

app/services/merge_requests/auto_merge_service.rb

@@ -6,7 +6,7 @@ module MergeRequests
   # Called when you do merge via GitLab UI
   class AutoMergeService < BaseMergeService
     def execute(merge_request, current_user, commit_message)
-      merge_request.lock
+      merge_request.lock_mr
 
       if Gitlab::Satellite::MergeAction.new(current_user, merge_request).merge!(commit_message)
         merge_request.merge
@@ -17,11 +17,11 @@ module MergeRequests
 
         true
       else
-        merge_request.unlock
+        merge_request.unlock_mr
         false
       end
     rescue
-      merge_request.unlock if merge_request.locked?
+      merge_request.unlock_mr if merge_request.locked?
       merge_request.mark_as_unmergeable
       false
     end