Secret detection analyze only in branch changes

What does this MR do?

Now if I develop on a branch B, then the new incoming changes to the default branch A also are analyzed for secrets. Probably it would be better to analyse only the changes in the branch B that are not present in A. With this MR only changes in B are analysed.

Related issues

Check-list

Pre-merge

Consider the effect of the changes in this merge request on the following:

• Different pipeline types
• Non-canonical projects:
  • gitlab-foss
  • security
  • dev
  • personal forks
• Pipeline performance

If new jobs are added:

• Change-related rules (e.g. frontend/backend/database file changes): _____
• Frequency they are running (MRs, main branch, nightly, bi-hourly): _____
• Add a duration chart to https://app.periscopedata.com/app/gitlab/652085/Engineering-Productivity---Pipeline-Build-Durations if there are new jobs added to merge request pipelines

This will help keep track of expected cost increases to the GitLab project average pipeline cost per merge request RPI

Post-merge

• Consider communicating these changes to the broader team following the communication guideline for pipeline changes