Fix comparison of objects in ClaimDiffer

ClaimDiffer was detecting changes in the mainsnak even if nothing was
changed. This was due to a wrong comparison method used for comparing snak objects.

REQUIRES: I2653426355 in DataModel to pass PHPSTRICT

Change-Id: I2f3273596731276770c66695ac839aa6912f133d
(cherry picked from commit 0990b878d20ddbf83b0e3649118119c47dcf284e)
Merge "Fix comparison of objects in ClaimDiffer" into mw1.23-wmf3