ltfs_ordered_copy: honor `--keep-tree` for single file copies

[softloft38p-michael]

Summary of changes

This pull request includes following changes or fixes.

• Add --keep-tree support for single file copies

Description

Single file copies did not honor the --keep-tree flag so would always end up in the destination folder regardless of tree depth of source. This changes makes it possible to copy a single file with the tree preserved. It reuses the same logic that is used for multi-file copies.

Type of change

• New feature (non-breaking change which adds functionality)

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have confirmed my fix is effective or that my feature works

[piste-jp]

@softloft38p-michael ,

Thank you for your PR! It looks a good improvement.