Update git tests to not use local partial clones (#12721)

This works around a git restriction to fix CVE-2024-32004.
Users affected by the error `remote: fatal: could not fetch ... from promisor remote`
should consider setting the GIT_NO_LAZY_FETCH=0 if they trust the source repo.

tests/lib/local_repos.py

@@ -37,7 +37,7 @@ def local_checkout(
         created as a sub directory of the base temp directory.
     """
     assert "+" in remote_repo
-    vcs_name = remote_repo.split("+", 1)[0]
+    vcs_name, vcs_url = remote_repo.split("+", 1)
     repository_name = os.path.basename(remote_repo)
 
     directory = temp_path.joinpath("cache")
@@ -51,6 +51,12 @@ def local_checkout(
         assert repository_name == "INITools"
         _create_svn_initools_repo(repo_url_path)
         repo_url_path = os.path.join(repo_url_path, "trunk")
+    elif vcs_name == "git":
+        # Don't use vcs_backend.obtain() here because we don't want a partial clone:
+        # https://github.com/pypa/pip/issues/12719
+        subprocess.check_call(
+            ["git", "clone", vcs_url, repo_url_path],
+        )
     else:
         vcs_backend = vcs.get_backend(vcs_name)
         assert vcs_backend is not None