Fix commit search on some elasticsearch indexes

What does this MR do?

Ensures that commit search works, even on elasticsearch indexes where the project rid is created as a string, rather than integer, field

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Commit search is broken on some indexes at the moment

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #2280 (closed)