Add concurrent indexes for by_head_commit sha query

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/148466



Merged-by: Alper Akgun <aakgun@gitlab.com>
Approved-by: João Pereira <jpereira@gitlab.com>
Approved-by: Alper Akgun <aakgun@gitlab.com>
Co-authored-by: Pam Artiaga <partiaga@gitlab.com>