Merge branch 'ab/retire-refs-unused-funcs'

Code cleanup. * ab/retire-refs-unused-funcs: refs/ref-cache.[ch]: remove "incomplete" from create_dir_entry() refs/ref-cache.c: remove "mkdir" parameter from find_containing_dir() refs/ref-cache.[ch]: remove unused add_ref_entry() refs/ref-cache.[ch]: remove unused remove_entry_from_dir() refs.[ch]: remove unused ref_storage_backend_exists()