Cells: Designate merge_request_diff_commit_users as clusterwide

In researching groupcode review tables and sharding (#440140 (comment 1800513637)), merge_request_diff_commit_users doesn't have any references to other tables or data structures, and in the code is just matched against emails.

@tigerwnz:

merge_request_diff_commit_users is an interesting one, there may be an argument for this table to be gitlab_main_clusterwide as it seems be essentially equivalent to users (which is itself clusterwide). If this was cell-local, we'd need to store duplicate data for any name/email that committed to projects belonging to more than one organisation.

@manojmj:

I am ok with merge_request_diff_commit_users being clusterwide. I do not see a way to attribute data in it to a cell-local entity 🤔