Convert merge request SHA columns to binary

Extracted from https://gitlab.com/gitlab-org/gitlab-ce/issues/31825:

• merge_commit_sha should be bytea
• rebase_commit_sha should be bytea
• in_progress_merge_commit_sha should be bytea

These columns can be made bytea to reduce the space necessary by 50%. This does however require that any columns these columns are joined onto are also converted to bytea.