Change git log commit range for secret detection

Why is this change being made?

When doing some investigation for gitlab-org/gitlab#349063 (closed) we discovered that the range of commits generated by git log in the secret_detection job has the potential to be wrong, and in some cases, time out the job.

This MR changes the ... operation to ... If we take main...feature as the example, the current ... syntax means "give us commits that are reachable from feature or main but not both". This means we could be including commits that are not associated with the MR when scanning for secrets. Using main..feature limits us to commits that are just in feature.

More reading:

• https://darekkay.com/blog/git-commit-ranges/
• https://git-scm.com/docs/git-log

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