Add levels for refs and symrefs

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"