Pack reconstructed dependency fixtures

Creating the historical smmap tag leaves a loose, read-only Git object in the
reconstructed repository. Local clones may hardlink loose objects, and Python
3.7 on Windows cannot remove those files during temporary-directory cleanup.

Run Git garbage collection after constructing each fixture so its generated
refs and objects use normal packed repository storage before tests clone it.

Author: codex

test/lib/helper.py

@@ -411,6 +411,7 @@ def setUpClass(cls):
             repo.create_head("master", repo.commit(rev), force=True).checkout()
             if name == "smmap":
                 repo.create_tag("v0.8.1", ref="master~10", message="Test fixture tag", force=True)
+            repo.git.gc()
             repo.close()
             cls._dependency_repo_dirs[name] = path