fix: replace deprecated git lfs clone with git clone + git lfs fetch --all

git lfs clone is deprecated - modern git clone handles LFS automatically.
Using git lfs fetch --all ensures all LFS objects across all refs are
backed up, matching the existing bare clone behavior and providing
complete LFS backups.

Closes #379

README.rst

@@ -215,6 +215,8 @@ When you use the ``--lfs`` option, you will need to make sure you have Git LFS i
 
 Instructions on how to do this can be found on https://git-lfs.github.com.
 
+LFS objects are fetched for all refs, not just the current checkout, ensuring a complete backup of all LFS content across all branches and history.
+
 
 About Attachments
 -----------------

github_backup/github_backup.py

@@ -2090,11 +2090,13 @@ def fetch_repository(
                     git_command.pop()
                 logging_subprocess(git_command, cwd=local_dir)
         else:
-            if lfs_clone:
-                git_command = ["git", "lfs", "clone", remote_url, local_dir]
-            else:
-                git_command = ["git", "clone", remote_url, local_dir]
+            git_command = ["git", "clone", remote_url, local_dir]
             logging_subprocess(git_command)
+            if lfs_clone:
+                git_command = ["git", "lfs", "fetch", "--all", "--prune"]
+                if no_prune:
+                    git_command.pop()
+                logging_subprocess(git_command, cwd=local_dir)
 
 
 def backup_account(args, output_directory):