Rework levels, and add some new ones

levels/symref-create/description

@@ -4,6 +4,4 @@ When that happens, they are called "symbolic refs". You can create or update a s
 
     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/"! You might already have seen it in the other levels - it's the only thing that's always there!
-
-Try pointing HEAD to the refs in this repository!
+Create a symbolic ref called "refs/rainbow"!

levels/symref-create/goal

@@ -1 +1,3 @@
-git symbolic-ref HEAD refs/best_commit
+BLOB=$(git hash-object -w --stdin)
+git update-ref refs/double "$BLOB"
+git symbolic-ref refs/rainbow refs/double

levels/symref-create/start

@@ -1,4 +0,0 @@
-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/symref-create/win

@@ -1 +1 @@
-git symbolic-ref HEAD | grep _commit
+git symbolic-ref refs/rainbow