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 submodules, 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.

Paths are anchored at GIT_REPO -- the value of
GIT_PYTHON_TEST_GIT_REPO_BASE, falling back to the source tree -- so any
setup already satisfying the existing rorepo-redirection contract
automatically satisfies the new test.

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

Author: EliahKagan

test/test_fixture_health.py

@@ -24,6 +24,8 @@
 
 import pytest
 
+from test.lib import GIT_REPO
+
 REPO_ROOT = Path(__file__).resolve().parent.parent
 
 # Directories git must trust for the test suite to operate normally. The
@@ -40,6 +42,19 @@
     ),
 ]
 
+# Submodule working trees that must be present and initialized for the test
+# suite to operate normally. Anchored at ``GIT_REPO`` (the value of
+# ``GIT_PYTHON_TEST_GIT_REPO_BASE``, falling back to the source tree) -- the
+# same path the rest of the test suite uses for its read-only ``rorepo``
+# fixture.
+SUBMODULE_DIRS = [
+    pytest.param(Path(GIT_REPO) / "git" / "ext" / "gitdb", id="gitdb"),
+    pytest.param(
+        Path(GIT_REPO) / "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 +97,26 @@ 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 fixture tree pointed at by
+    ``GIT_PYTHON_TEST_GIT_REPO_BASE`` (or the source tree, if the env var is
+    unset) is missing this submodule's working tree -- typically because
+    ``init-tests-after-clone.sh`` (or its equivalent) wasn't run.
+    """
+    assert submodule_dir.is_dir(), (
+        f"Submodule working tree missing: {submodule_dir}.\n"
+        "Run ./init-tests-after-clone.sh from the repo root, or set "
+        "GIT_PYTHON_TEST_GIT_REPO_BASE to a tree where submodules have been "
+        "initialized recursively."
+    )
+    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."
+    )