commit ba00e640f658ad8d0a4dff09a497a51b8a4de935
Author: Herman Lee <herman@fb.com>
Date:   Wed Feb 22 06:30:06 2017 -0800

    Improve add_index_alter_cardinality test

    Summary:
    Split add_index_inplace_cardinality test out and add a debug_sync point
    to it so that the flush of the memtable occurs while the alter is
    running.
    Closes https://github.com/facebook/mysql-5.6/pull/539

    Reviewed By: alxyang

    Differential Revision: D4597887

    Pulled By: hermanlee

    fbshipit-source-id: faedda2