fix confusing 'no such command _id' message from rollback_fake_cmd.js output

rollback_fake_cmd.js tests the server's ability to rollback an oplog entry containing a nonexistent command name - in the current test script, it reuses an oplog entry from a collection insertion as the command object which leads to the following confusing test output:

2016-02-06T06:51:43.333+0000 I -        [rsBackgroundSync] Fatal assertion 28723 UnrecoverableRollbackError: rollback no such command _id @ 18751