Add levels for refs and symrefs

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