A Wikimedia community member has beat me to do a pull request before I do: https://github.com/MariaDB/server/pull/300

The proximate cause of this issue is that we (Phabricator) began moving FULLTEXT indexes to the InnoDB table engine on database servers that support InnoDB FULLTEXT in early December. You can find some context in the Phabricator upstream, here:

https://secure.phabricator.com/T11741

This engine change happens automatically during the routine upgrade process, which also runs other migrations and schema adjustments. Any install running MariaDB 10.0 with a reasonable amount of data is likely to be impacted by this issue, since we index some exotic data like commit messages which probably contain tokens with lengths greater than 127 bytes in any nontrivial repository.

Users don't explicitly choose to perform this engine migration, so impacted users probably will not have an easy path back to safety. We could explicitly blacklist MariaDB 10.0 and prevent it from upgrading to InnoDB FULLTEXT to work around this, but a MariaDB upstream fix would obviously be preferable.

Thank you for the report. I cherry-picked my MDEV-11233 patch to the 10.0 branch.
I should have realized that also 10.0 is affected, because both are based on the MySQL 5.6, and the MySQL 5.6 source code changes are first merged to 10.0, then from 10.0 to 10.1.
As far as I can tell, the bug was introduced by an invalid merge conflict resolution when merging MySQL 5.6 changes.