insights: commit indexer fails for repository due to duplicate key violation

Created by: chwarwick

The follow error has been reported:

t=2022-04-26T19:17:05+0000 lvl=eror msg="unable to update commit index repo_id: {REPO_ID}: error saving commit for repo_id: {REPO_ID} commit_id {COMMIT}: ERROR: duplicate key value violates unique constraint \"commit_index_pkey\" (SQLSTATE 23505)"

Commit index table schema:

create table commit_index
(
    committed_at timestamp with time zone not null,
    repo_id      integer                  not null,
    commit_bytea bytea                    not null,
    indexed_at   timestamp default CURRENT_TIMESTAMP,
    debug_field  text,
    primary key (committed_at, repo_id, commit_bytea)
)```

This error will block the insights indexer from indexing additional commits for the repository and will cause insights to fall back to uncompressed query plans.