Moved to brute-force deorphan without parent pointers

Removing the dependency to the parent pointer solves
many issues with non-atomic updates of children's
parent pointers with respect to any move operations.

However, this comes with an embarrassingly terrible
runtime as the only other option is to exhaustively
check every dir entry to find a child's parent.

Fortunately, deorphaning should be a relatively rare
operation.

tests/test_orphan.sh

@@ -0,0 +1,41 @@
+#!/bin/bash
+set -eu
+
+echo "=== Orphan tests ==="
+rm -rf blocks
+tests/test.py << TEST
+    lfs_format(&lfs, &config) => 0;
+TEST
+
+echo "--- Orphan test ---"
+tests/test.py << TEST
+    lfs_mount(&lfs, &config) => 0;
+    lfs_mkdir(&lfs, "parent") => 0;
+    lfs_mkdir(&lfs, "parent/orphan") => 0;
+    lfs_mkdir(&lfs, "parent/child") => 0;
+    lfs_remove(&lfs, "parent/orphan") => 0;
+TEST
+# remove most recent file, this should be the update to the previous
+# linked-list entry and should orphan the child
+rm -v "blocks/$(ls -t blocks | sed -n '/^[0-9a-f]*$/p' | sed -n '1p')"
+tests/test.py << TEST
+    lfs_mount(&lfs, &config) => 0;
+    lfs_stat(&lfs, "parent/orphan", &info) => LFS_ERROR_NO_ENTRY;
+    unsigned before = 0;
+    lfs_traverse(&lfs, test_count, &before) => 0;
+    test_log("before", before);
+
+    lfs_deorphan(&lfs) => 0;
+
+    lfs_stat(&lfs, "parent/orphan", &info) => LFS_ERROR_NO_ENTRY;
+    unsigned after = 0;
+    lfs_traverse(&lfs, test_count, &after) => 0;
+    test_log("after", after);
+
+    int diff = before - after;
+    diff => 2;
+    lfs_unmount(&lfs) => 0;
+TEST
+
+echo "--- Results ---"
+tests/stats.py