git_helper.py is spawned as a standalone subprocess on Windows
(manager_core.py:1342). The import of comfyui_manager.common.timestamp_utils
triggered comfyui_manager/__init__.py which imports from comfy.cli_args —
unavailable in the subprocess environment, causing ModuleNotFoundError.
Inline get_backup_branch_name() using only stdlib (time, uuid), preserving
the original collision-detection and UUID-fallback semantics. Add 9 tests
covering standalone subprocess loading, behavioral equivalence, edge cases
(repo.heads exception, 99-suffix exhaustion with UUID fallback).
