Revert a default GIT_DEPTH for MR pipeline

What does this MR do?

Revert the DEFAULT_GIT_DEPTH_MERGE_REQUEST value for git_depth that was introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25504

Implementation idea from: https://gitlab.com/gitlab-org/gitlab-ce/issues/62418#note_176073801

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content