shelve: move unknown files handling to a separate function This change has nothing to do with future obsshelve introduction, it is done just for readability purposes.

shelve: move actual created commit shelving to a separate function Currently, this code does not have any branching, it just bundles a commit and saves a patch file. Later, obsolescence-based shelve will be added, so this code will also create some obsmarkers and will be one of the few places where obsshelve will be different from traditional shelve.

shelve: move 'nothing changed' messaging to a separate function This has nothing to do with the future obsshelve implementation, I just thought that moving this messaging to a separate function will improve shelve code readability.