Scan the result of merge instead of diff

[Wovchena]

Is your feature request related to a problem? Please describe.
When a dependency is added and dependency-review-action is enabled after, the dependency isn't scanned.

Describe the solution you'd like
dependency-review-action should scan the result of PR merge by default.

[github-actions]

👋 This issue has been marked as stale because it has been open with no activity for 180 days. You can: comment on the issue or remove the stale label to hold stalebot off for a while, add the Keep label to hold stale off permanently, or do nothing. If you do nothing, this issue will be closed eventually by the stalebot. Please see CONTRIBUTING.md for more policy details.

[Wovchena]

Keep