fixup! erp5_core: Make two queries in Base_getRelatedDocumentList

My suggestion of using sorted in
!1373 (comment 129103) was wrong,
ERP5 document do not sort in deterministic order. Use a key function to make
sure we get results in same order
11 jobs for for_testrunner_1 in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

passed ERP5.CodingStyleTest-TestRunner1

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passed ERP5.PerformanceTest-Master

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passed ERP5.PerformanceTest-TestRunner1

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failed ERP5.UnitTest-Master

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failed ERP5.UnitTest-Master.Medusa

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failed ERP5.UnitTest-TestRunner1

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passed SlapOS.Eggs.UnitTest-Master.Python2

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passed SlapOS.Eggs.UnitTest-Master.Python3

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passed SlapOS.Eggs.UnitTest-Master.Python2

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passed SlapOS.Eggs.UnitTest-Master.Python3

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