Add a check that required submodules are initialized

Adds test_required_submodule_is_initialized to test/test_fixture_health.py.
For each of the gitdb and smmap submodule working trees -- anchored at
the GitPython source tree, since submodule paths are recorded in
.gitmodules and are properties of the source tree itself -- asserts
that the working tree directory exists and contains a .git marker
(file or directory). Failure prints an actionable message naming
init-tests-after-clone.sh as the remedy.

This is a regression test for a different contract than the existing
test_fixture_dir_is_trusted_by_git in this module. That test verifies
git's willingness to operate in each fixture directory (the
safe.directory / dubious-ownership contract). The new test verifies
that the directories are populated at all (the init-tests-after-clone.sh
contract). A bug class along these lines affected Arch Linux in gitpython-developers#1713
when init-tests-after-clone.sh stopped running git submodule update on
CI -- if a similar regression returns, this test catches it loudly with
a message naming the cause, instead of producing a cascade of opaque
downstream failures.

The test skips when the source tree is not a git clone (no .git at
REPO_ROOT). This accommodates setups that run pytest from an extracted
release tarball and prepare submodules in a separately-pointed tree
via GIT_PYTHON_TEST_GIT_REPO_BASE (e.g. OpenIndiana's package build):
in such setups, `git submodule update` cannot operate on the source
tree, and the submodule paths checked here would never be populated
there -- but the test suite still works because rorepo points
elsewhere.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: EliahKagan

test/test_fixture_health.py

@@ -40,6 +40,19 @@
     ),
 ]
 
+# Submodule working trees that must be present and initialized for the test
+# suite to operate normally. Anchored at the source tree (where this test
+# file lives), since submodule paths are properties of the GitPython source
+# tree itself -- recorded in ``.gitmodules`` -- and not of any rorepo
+# redirection target.
+SUBMODULE_DIRS = [
+    pytest.param(REPO_ROOT / "git" / "ext" / "gitdb", id="gitdb"),
+    pytest.param(
+        REPO_ROOT / "git" / "ext" / "gitdb" / "gitdb" / "ext" / "smmap",
+        id="smmap",
+    ),
+]
+
 
 @pytest.mark.parametrize("fixture_dir", FIXTURE_DIRS)
 def test_fixture_dir_is_trusted_by_git(fixture_dir: Path) -> None:
@@ -82,3 +95,32 @@ def test_fixture_dir_is_trusted_by_git(fixture_dir: Path) -> None:
         "This usually means the directory is not an initialized git "
         "repository (its `.git` marker may be stale or pointing elsewhere)."
     )
+
+
+@pytest.mark.parametrize("submodule_dir", SUBMODULE_DIRS)
+def test_required_submodule_is_initialized(submodule_dir: Path) -> None:
+    """The vendored submodule's working tree is present and initialized.
+
+    Failure means the source tree is a git clone but ``init-tests-after-clone.sh``
+    (or its equivalent) hasn't been run, so the submodule's working tree has
+    not been populated. Skipped when the source tree itself isn't a git clone
+    (e.g. an extracted release tarball), since ``git submodule update`` cannot
+    operate there; setups that handle submodules in a separately-prepared
+    tree (via ``GIT_PYTHON_TEST_GIT_REPO_BASE``) are exempted from this check.
+    """
+    if not (REPO_ROOT / ".git").exists():
+        pytest.skip(
+            "Source tree is not a git clone (no .git in REPO_ROOT); submodules "
+            "cannot be initialized via `git submodule update` here. Setups "
+            "that prepare submodules in a separately-pointed tree (via "
+            "GIT_PYTHON_TEST_GIT_REPO_BASE) are exempted from this check."
+        )
+    assert submodule_dir.is_dir(), (
+        f"Submodule working tree missing: {submodule_dir}.\nRun ./init-tests-after-clone.sh from the repo root."
+    )
+    assert (submodule_dir / ".git").exists(), (
+        f"Submodule directory exists but has no .git marker: {submodule_dir}.\n"
+        "This usually means the directory was created (e.g. by extracting a "
+        "source tarball) but `git submodule update --init` was never run for "
+        "it. Run ./init-tests-after-clone.sh from the repo root."
+    )