gitclone: support git 2.38.1 when using submodules

See https://lore.kernel.org/lkml/xmqq4jw1uku5.fsf@gitster.g/

In the case of slapos, cloned repositories are considered trusted, so
it should not be an issue to set this flag.
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Jerome.Python2

00:54:20

running SlapOS.Eggs.UnitTest-Jerome.Python2

17920:39:34

passed SlapOS.Eggs.UnitTest-Jerome.Python3

00:43:06

running SlapOS.Eggs.UnitTest-Jerome.Python3

17920:39:07

running SlapOS.Eggs.UnitTest-Master.Python2

17920:31:31

running SlapOS.Eggs.UnitTest-Master.Python3

17920:32:41

passed SlapOS.Eggs.UnitTest-Jerome.Python2

00:11:53

passed SlapOS.Eggs.UnitTest-Jerome.Python3

00:12:54