Fix assertion when gmail labels nest; fix #6304
Fix assertion when gmail labels nest; fix #6304

We want to use the normal folder comparator by default, and the special
comparator only in the root level.

tshikaboom/geary,mjog/geary,mjog/geary,robol/geary,rschroll/geary,AtticHacker/geary,kyoushuu/geary,githubnemo/geary,AtticHacker/geary,jimbonator/geary,mjog/geary,aatishnn/geary,jimbonator/geary,tshikaboom/geary,aatishnn/geary,geekvigilante/geary,robol/geary,mpdeimos/geary,abn/geary,martinra/geary,glebihan/geary,emersion/geary,MohamedIM/geary-unofficial,rschroll/geary,aatishnn/geary,kyoushuu/geary,robol/geary,githubnemo/geary,glebihan/geary,martinra/geary,AtticHacker/geary,jimbonator/geary,tshikaboom/geary,emersion/geary,githubnemo/geary,kpiwko/geary,martinra/geary,glebihan/geary,emersion/geary