Membership Detail report suppresses contact name if two contacts share a name

The function alterDisplay() in the Membership Detail report suppresses repeating names - even if the names belong to different contacts. I'm submitting a patch that suppresses Display Name based on the Contact ID instead.

While testing this alongside CRM-15917, I also found that Display Names are suppressed even if one row doesn't directly follow the next row - which didn't happen before CRM-15917. This patch implements the behavior described by the pre-existing comment - suppress display names only if the row(s) immediately above contains the same contact ID.