The DataMapper should check if an object is already in the identityMap
before calling fetchRelated, since otherwise the object will be retrieved
from the database again.

Resolves: #53514
Releases: 6.1,6.2
Change-Id: I24e262322f1f0ba3c346fa01c50fa9063866aef7
Reviewed-on: https://review.typo3.org/25252
Reviewed-by: Nico de Haen
Reviewed-by: Fabien Udriot
Tested-by: Wouter Wolters
Reviewed-by: Steffen Müller
Tested-by: Steffen Müller