Have init script clone submodules unconditionally

Since 7110bf8 (in #1693), "git submodule update --init --recursive"
was not run on CI, on the mistaken grounds that the CI test
workflows would already have taken care of cloning all submodules
(ever since 4eef3ec when the "submodules: recursive" option was
added to the actions/checkout step).

This changes the init-tests-after-clone.sh script to again run that
command unconditionally, including on CI. The assumption that it
wasn't needed on CI was based on the specific content of
GitPython's own GitHub Actions workflows. But this disregarded that
the test suite is run on CI for *other* projects: specifically, for
downstream projects that package GitPython (#1713).

This also brings back the comment from fc96980 that says more about
how the tests rely on submodules being present (specifically, that
they need a submodule with a submodule). However, that is not
specifically related to the bug being fixed.

init-tests-after-clone.sh

@@ -47,10 +47,8 @@ git reset --hard HEAD~1
 # Point the master branch where we started, so we test the correct code.
 git reset --hard __testing_point__
 
-# The tests need submodules. (On CI, they would already have been checked out.)
-if ! ci; then
-    git submodule update --init --recursive
-fi
+# The tests need submodules, including a submodule with a submodule.
+git submodule update --init --recursive
 
 # The tests need some version tags. Try to get them even in forks. This fetches
 # other objects too. So, locally, we always do it, for a consistent experience.