Add levels for refs and symrefs

levels/00-empty/description

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+This is an empty level without any winning condition - you can use it as a playground!

levels/05-ref/description

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+Let's take a look at "refs", short for "references"! Refs are not objects, but rather very simple *pointers* to objects! They can help you keep track of what's where.
+
+You can create or update a ref with
+
+    git update-ref refs/<refname> <newvalue>
+
+Make sure to start a ref's name with "refs/"! That's a convention that helps Git find all refs you create.
+
+Create refs that point to all objects in this repository!

levels/05-ref/start

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+echo hello > hello
+echo world > world
+git add .
+TREE=$(git write-tree)
+git commit-tree $TREE -m "Initial commit"

levels/05-ref/win

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+OBJECTS=$(git cat-file --batch-check='%(objectname)' --batch-all-objects | sort)
+REF_TARGETS=$(git show-ref -s | sort | uniq)
+test "$OBJECTS" = "$REF_TARGETS"

levels/06-symref/description

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+Instead of pointing directly to objects, refs can also point to other refs!
+
+When that happens, they are called "symbolic refs". You can create or update a symbolic ref using
+
+    git symbolic-ref <name> <ref>
+
+Usually, you will only encounter a special symbolic ref called "HEAD". This ref is special in that it doesn't start with "refs/"!
+
+Try pointing HEAD to the refs in this repository!

levels/06-symref/start

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+TREE=$(git write-tree)
+COMMIT=$(git commit-tree $TREE -m "Initial commit")
+git update-ref refs/best_commit $COMMIT
+git update-ref refs/worst_commit $COMMIT

levels/06-symref/win

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+git symbolic-ref HEAD | grep _commit