Add concurrent indexes for by_head_commit sha query

What does this MR do and why?

Issue: Create concurrent indexes for fast-forward MRs ... (#455124 - closed)

This index is for the logic introduced in Insert MRs by head_commit_sha separately (!147297 - merged). The asynchronous indexes were also added in that other MR.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

I've confirmed that the indexes are already added in gprd through the async index creation:

How to set up and validate locally

Run the db migration:

bundle exec rails db:migrate