git: Drop POSIXPERM for the tests

Shared permissions are forbidden in the Nix build sandbox which is why
some of the tests fail. Dropping the POSIXPERM test prerequisite, that
is added unconditionally on Linux, should help us avoiding uninteresting
test failures after Git updates. The drawback is that this disables a
few additional tests (as of Git 2.32.0 approx. 71 out of more than 20k
subtests require POSIXPERM).

pkgs/applications/version-management/git-and-tools/git/default.nix

@@ -290,11 +290,13 @@ stdenv.mkDerivation {
       fi
     }
 
-    # Shared permissions are forbidden in sandbox builds.
-    disable_test t0001-init shared
+    # Shared permissions are forbidden in sandbox builds:
+    substituteInPlace t/test-lib.sh \
+      --replace "test_set_prereq POSIXPERM" ""
+    # TODO: Investigate while these still fail (without POSIXPERM):
+    disable_test t0001-init 'shared overrides system'
+    disable_test t0001-init 'init honors global core.sharedRepository'
     disable_test t1301-shared-repo
-    disable_test t5324-split-commit-graph 'split commit-graph respects core.sharedrepository'
-    disable_test t4129-apply-samemode 'do not use core.sharedRepository for working tree files'
 
     # Our patched gettext never fallbacks
     disable_test t0201-gettext-fallbacks