InnoDB was writing unnecessary information to the
update undo log records. Most notably, if an indexed column is updated,
the old value of the column would be logged twice: first as part of
the update vector, and then another time because it is an indexed column.

Because the InnoDB undo log record must fit in a single page,
this would cause unnecessary failure of certain updates.

Even after this fix, InnoDB still seems to be unnecessarily logging
indexed column values for non-updated columns. It seems that non-updated
secondary index columns only need to be logged when a PRIMARY KEY
column is updated. To reduce risk, we are not fixing this remaining flaw
in GA versions.

trx_undo_page_report_modify(): Log updated indexed columns only once.