Querying merge requests using the merge commit SHA is slow

Discovered in https://gitlab.com/gitlab-org/gitlab-ce/issues/37814

When displaying a merge request we end up running a query along the lines of the following:

SELECT "merge_requests".*
FROM "merge_requests"
WHERE "merge_requests"."deleted_at" IS NULL
AND "merge_requests"."target_project_id" = 13083
AND "merge_requests"."merge_commit_sha" = 'e80a893ff0ea8466099f6478183631af55933db2'
ORDER BY "merge_requests"."id" DESC
LIMIT 1;

Because merge_commit_sha is not indexed we end up running a Filter instead of a regular index scan. Further we have to perform a Sort, instead of a backwards index scan.

To improve this we'd have to:

1. Get rid of the deleted_at filter
2. Add an index on (target_project_id, merge_commit_sha, id)
3. Remove the standalone index on target_project_id since queries using said index can use the one we added in the previous step There is no such index

See https://gitlab.com/gitlab-org/gitlab-ce/issues/37814#note_40870711 for more information.