Prepare asynchronous index creation for project_id

What does this MR do and why?

As part of https://gitlab.com/groups/gitlab-org/-/epics/16906+, there is some initial setup needed to create a primary key for merge_request_diff_commits: https://gitlab.com/gitlab-org/gitlab/-/issues/528420+. This is part 2 of 3 (see below) where we're preparing to create an async index for project_id.

Changelog: performance

MR	Description
!196884 (merged)	Add project_id column to old (current) merge_request_diff_commits table
!196931 (merged) 📍	Prepare asynchronous index creation for project_id
!197061 (merged)	Create project_id index synchronously

References

• https://gitlab.com/gitlab-org/gitlab/-/issues/528420+
• https://gitlab.com/groups/gitlab-org/-/epics/16906+
• https://gitlab.com/gitlab-org/architecture/merge-request-diff-commits-deduplication/-/blob/main/README.md

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