delete: preserve empty dirs in --backup-dir when deleting

[x15sr71]

Description

When using --backup --backup-dir --delete, rsync was silently removing empty directories from the destination instead of preserving them in --backup-dir. Files in those directories were backed up correctly, but the empty directories themselves were not.

Root cause

delete_item() routed S_ISDIR directly to do_rmdir_at(), bypassing the make_backups/backup_dir logic that only existed in the non-directory else branch.

Fix

Fix attempt a rename() into backup-dir before rmdir(). If the destination already exists (because child file backups already created that directory in backup-dir, as noted by @steadytao in the issue), fall through to rmdir() of the now-empty source. If rename() fails for any other unexpected reason (e.g. EXDEV), emit a warning and fall through to rmdir().

Fixes #842.

Tests added in testsuite/backup-empty-dir_test.py:

• Phase 1: basic --backup does not keep empty directories in --backup-dir #842 reproducer — empty dir appears in backup-dir
• Phase 2: collision safety — existing backup-dir/sub/ from child file
  backups is not destroyed when parent dir is later backed up
• Phase 3: nested empty directories (a/b/c)
• Phase 4: no regression without --backup-dir — empty dir still deleted
• Phase 5: --delete-delay variant — backup path reached for queued deletions

[tridge]

@x15sr71 thanks for the PR! There is a subtle bug with it in the handling of files that should not go into the backup dir. I've added another test to the PR that demonstrates the bug