fix(git_helper): remove comfyui_manager import to fix Windows subprocess crash (#2703)

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).

comfyui_manager/common/git_helper.py

@@ -9,7 +9,6 @@ import yaml
 import requests
 from tqdm.auto import tqdm
 from git.remote import RemoteProgress
-from comfyui_manager.common.timestamp_utils import get_backup_branch_name
 
 
 comfy_path = os.environ.get('COMFYUI_PATH')
@@ -68,6 +67,36 @@ class GitProgress(RemoteProgress):
         self.pbar.refresh()
 
 
+def get_backup_branch_name(repo=None):
+    """Get backup branch name with current timestamp.
+
+    Inlined from timestamp_utils to keep git_helper.py standalone — this script
+    runs as a subprocess on Windows and must not import from comfyui_manager.
+    """
+    import time as _time
+    import uuid as _uuid
+
+    base_name = f'backup_{_time.strftime("%Y%m%d_%H%M%S")}'
+
+    if repo is None:
+        return base_name
+
+    try:
+        existing_branches = {b.name for b in repo.heads}
+    except Exception:
+        return base_name
+
+    if base_name not in existing_branches:
+        return base_name
+
+    for i in range(1, 100):
+        new_name = f'{base_name}_{i}'
+        if new_name not in existing_branches:
+            return new_name
+
+    return f'{base_name}_{_uuid.uuid4().hex[:6]}'
+
+
 def gitclone(custom_nodes_path, url, target_hash=None, repo_path=None):
     repo_name = os.path.splitext(os.path.basename(url))[0]