Incorrect PSI_stage_info message in SHOW PROCESSLIST during parallel replication

In parallel replication, threads can do two different waits for a prior
transaction. One is for the prior transaction to start commit, the other is
for it to complete commit.

It turns out that the same PSI_stage_info message is errorneously used in
both cases (probably a merge error), causing SHOW PROCESSLIST to be
misleading.