Rename chapters into "internals" and "intro"

levels/internals/tree-read/description

@@ -0,0 +1,7 @@
+As soon as you have some tree objects, you can always read them and set the index exactly to their content! Unsurprisingly, the command is called
+
+    git read-tree <tree>
+
+For <tree>, you can provide the hash of any tree object - you can right-click one to insert its hash into the terminal!
+
+Try reading some of the trees in this repository into the index!

levels/internals/tree-read/goal

@@ -0,0 +1 @@
+git read-tree "$TRIPLE_TREE"

levels/internals/tree-read/start

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+EMPTY_TREE=$(git write-tree)
+
+echo "file 1" > file1
+echo "file 2" > file2
+git add .
+git write-tree
+
+rm *
+echo "file A" > fileA
+echo "file B" > fileB
+echo "file C" > fileC
+git add .
+TRIPLE_TREE=$(git write-tree)
+
+git read-tree "$EMPTY_TREE"

levels/internals/tree-read/win

@@ -0,0 +1 @@
+test "$(git ls-files | wc -l)" -gt 0