GitLab: trigger builds for MR on code changes only

[stsewd]

What's the problem this feature will solve?

Currently, we are triggering a build when an MR changed, like even when the title changed. But we want to trigger builds only when a code change was made.

Describe the solution you'd like

As the documentation suggests https://docs.gitlab.com/ee/user/project/integrations/webhook_events.html#merge-request-events

The field object_attributes.oldrev is only available when there are actual code changes, like:

We should in addition check for that attribute when the action is update.

Additional context

We are triggering builds for MR even when a title is changed.