Extract terminal in its own scene, add a few proper levels with a description

levels/01-blob/description

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+At its core, Git is very simple. It stores "objects", which are basically files identified by an "identifier" (short: ID).
+
+There are four types of objects: blobs, trees, commits, and tags. The simplest type is a "blob", which is just a piece of text.
+
+Let's create some blobs! To do that, create a file with the desired content, and then use
+
+    $ git hash-object -w <file>
+
+The flag -w means "write", and tells Git to actually write the new blob to the disk.
+
+Create three new blobs!

levels/02-tree/description

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+Blobs just have some content and an ID. It would be convenient to be able to give names to a blob, right?
+
+The second type of Git object is called a "tree" - a tree points a bunch of blobs, and gives each of them a name!
+
+How can we build our own trees? Introducing: the index!
+
+The index is like a "work in progress" version of a tree, where we can add and remove blobs. When we're happy with what we have built, we can make it into a real tree object!
+
+We can use
+
+    $ git update-index --add <file>
+
+to make the file's content into a blob, and to add that blob to the index using the file's name.
+
+When we're happy with out index, we can convert it into a tree using
+
+    $ git write-tree
+
+That command will output the ID of the newly crated tree.
+
+Build a tree containing just the 'noises' file, which we conveniently placed in your current directory! :)

levels/02-tree/goal

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+echo 'meow' > noises
+git update-index --add noises
+git write-tree
+
+rm noises
+git update-index --remove noises

levels/02-tree/start

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+echo 'meow' > noises

levels/03-commit/description

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+So a tree describes the state of a directory structure at a specific point in time.
+
+It would be nice if we could remember when that state existed, and who authored it, right?
+
+Enter: commits. They are objects that point to a tree and contain some additional metadata. You can create a commit using
+
+    $ git commit-tree <tree> -m "Description of your commit"
+
+Make a commit from the tree in this repository!

levels/03-commit/goal

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+touch empty_file
+git add .
+git write-tree
+git commit-tree 3185 -m 'Clever commit message'
+
+rm empty_file
+git update-index --remove empty_file

levels/03-commit/start

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+touch empty_file
+git add .
+git write-tree
+
+rm empty_file
+git update-index --remove empty_file

levels/04-parents/description

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+When using the commit-tree command, you can optionally specify a parent:
+
+    $ git commit-tree <tree> -m "Description" -p <parent commit>
+
+Can you make a string of three commits? We placed an empty tree in your repo to get you started.

levels/04-parents/goal

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+git write-tree
+git commit-tree 4b82 -m 'We cannot really construct the goal yet :)'

levels/04-parents/start

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+git write-tree

levels/first/goal

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-git commit --allow-empty -m "Emtpy commit"

levels/first/start

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-echo -e "bread\ntomato\nbread" > sandwich
-git add sandwich
-git commit -m "Initial sandwich"
-echo -e "bread\ntomato\ncucumber\nbread" > sandwich
-git commit -am "Add cucumber"