[BUGFIX] Check identity map for existing objects

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