Mysql2::Error: Data too long for column 'st_diffs'

I just got the following error on this build https://gitlab.com/gitlab-org/gitlab-ce/builds/4716875

     # ------------------
     # --- Caused by: ---
     # Mysql2::Error:
     #   Data too long for column 'st_diffs' at row 1
     #   ./config/initializers/connection_fix.rb:20:in `execute'

which I can't reproduce locally since I'm using Postgre

/cc @smcgivern @stanhu