Fix a bug with thread collapsing.
Fix a bug with thread collapsing.

svn path=/trunk/kdenetwork/kmail/; revision=89822

lefou/kdepim-noakonadi,lefou/kdepim-noakonadi,lefou/kdepim-noakonadi,lefou/kdepim-noakonadi,lefou/kdepim-noakonadi,lefou/kdepim-noakonadi