Database updater produces a broken board

1. Install 3.0.10 via QI.
2. rsync 3.1 tree over the installation.
3. Run database updater - it succeeds.
4. Go to board index:

General Error
Could not get style data

BACKTRACE

FILE: (not given by php)
LINE: (not given by php)
CALL: msg_handler()

FILE: [ROOT]/includes/user.php
LINE: 185
CALL: trigger_error()

FILE: [ROOT]/index.php
LINE: 24
CALL: phpbb_user->setup()

Broken as of 7e2f16aafa47f8db51a12a293b0616cb75b4d12f, 398a6c8045113f14900bdea8c70ba032f75b45f6 produces a parse error, d4f1bdea3bdd27f7c9f99d6121b2762482fc1707 works - via `git log --merges phpBB/install/database_update.php`.

Clearing the cache makes no difference.

Reproduced with a manual 3.0.10 install database-updated to 3.1 also.