Allow merge commits for SD

What does this MR do and why?

Fixes an issue that prevents secret detection from being ran on merge commits for non-default branches due to the --no-merges flag. Removing the --no-merges flag fixes this issue. See the demo links below.

Describe in detail what your merge request does and why.

Screenshots or screen recordings

• Example MR: zrice/zd263681!4 (merged)
• Example job: https://gitlab.com/zrice/zd263681/-/pipelines/488503680 (note that this says 2 commits are being scanned, that is because the non-merge commit and merge commit are being scanned)

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

• Copy/paste https://gitlab.com/zrice/zd263681/-/raw/notmain/.gitlab-ci.yml into .gitlab-ci.yml for any test project
• Follow steps described here #351976 (comment 867763568)

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.