Two new low-level levels

levels/tree-nested/description

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+Trees can also point to other trees! This way, they can basically describe nested directory structures.
+
+When you add a file inside of a directory to the index, and then call `git write-tree`, it will automatically do the correct thing.
+
+To solve this level, build a little stick figure, as shown on the left - a tree that points to two blobs, and to a tree that points to two blobs.

levels/tree-nested/goal

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+echo "I'm the left arm" > arm1
+echo "I'm the right arm" > arm2
+mkdir hip
+echo "I'm the left leg" > hip/leg1
+echo "I'm the right leg" > hip/leg2
+git add .
+git write-tree

levels/tree-nested/win

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+TREES=$(git cat-file --batch-check='%(objectname) %(objecttype)' --batch-all-objects | grep tree | cut -f1 -d" ")
+
+for OUTER_TREE in $TREES; do
+    NUMBER_OF_BLOB_CHILDREN=$(git cat-file -p $OUTER_TREE | cut -f2 -d" " | grep blob | wc -l)
+    NUMBER_OF_TREE_CHILDREN=$(git cat-file -p $OUTER_TREE | cut -f2 -d" " | grep tree | wc -l)
+
+    if [ $NUMBER_OF_BLOB_CHILDREN -eq 2 -a $NUMBER_OF_TREE_CHILDREN -eq 1 ]; then
+        TREE_CHILD=$(git cat-file -p $OUTER_TREE | cut -f1 | grep tree | cut -d" " -f3)
+        NUMBER_OF_BLOB_CHILDREN_OF_TREE_CHILD=$(git cat-file -p $TREE_CHILD | cut -f2 -d" " | grep blob | wc -l)
+        if [ $NUMBER_OF_BLOB_CHILDREN_OF_TREE_CHILD -eq 2 ]; then
+            exit 0
+        fi
+    fi
+done
+
+exit 1