CRM-11895 civicrm allows contacts to be reserved more than once for the same survey

It only happens in a particular use case:

• Click Campaign -> Reserve Respondents
• Select a Group and click Search
• Note the number of contacts returned (say it's 100)
• Select one member of the group and reserve that member
• You are returned to the search screen, but now the total number of contacts is 99 (which is correct, since one of the members of the group is already reserved)
• Now, click the All records option and reserve all the records returned

Instead of getting a message saying 99 records selected, it says 100 records are selected. And when I reserve them, all 100 people in the group are reserved, including the 1 contact I had previously reserved (which now has two reservation activities).

I've attached a patch, but the patch is a desperate attempt to get it working without really understanding the bigger picture, so a careful review would be great (and it probably needs to be re-written).

Thanks!