Refactor Level class so that we don't need to copy shell scripts

levels/bottom-up/commit-parents/win

@@ -3,8 +3,8 @@ COMMITS=$(git cat-file --batch-check='%(objectname) %(objecttype)' --batch-all-o
 for COMMIT in $COMMITS; do
     echo a commit named $COMMIT
     if [ $(git rev-list $COMMIT | wc -l) -ge 3 ]; then
-        exit 0
+        return 0
     fi
 done
 
-exit 1
+return 1

levels/bottom-up/commit-rhombus/win

@@ -3,8 +3,8 @@ COMMITS=$(git cat-file --batch-check='%(objectname) %(objecttype)' --batch-all-o
 for COMMIT in $COMMITS; do
     # My first parent's parents has to be the same as my second parent's parent.
     if [ "$(git rev-parse --verify -q $COMMIT^1^)" = "$(git rev-parse --verify -q $COMMIT^2^)" ]; then
-        exit 0
+        return 0
     fi
 done
 
-exit 1
+return 1

levels/bottom-up/puzzle-trees-all-the-way-down/win

@@ -8,11 +8,11 @@ for TREE in $TREES; do
             if [ "$(git cat-file -p $TREE2 | wc -l)" -eq 1 ]; then
                 if [ "$(git cat-file -p $TREE2 | cut -f1 | grep tree | wc -l)" -eq 1 ]; then
                     # Same for its child! \o/
-                    exit 0
+                    return 0
                 fi
             fi
         fi
     fi
 done
 
-exit 1
+return 1

levels/bottom-up/symref-no-deref/win

@@ -1,2 +1,2 @@
-git symbolic-ref refs/fries && exit 1
+git symbolic-ref refs/fries && return 1
 test "$(git show-ref -s refs/fries)" = "035e2968dafeea08e46e8fe6743cb8123e8b9aa6"

levels/bottom-up/tree-nested/win

@@ -8,9 +8,9 @@ for OUTER_TREE in $TREES; do
         TREE_CHILD=$(git cat-file -p $OUTER_TREE | cut -f1 | grep tree | cut -d" " -f3)
         NUMBER_OF_BLOB_CHILDREN_OF_TREE_CHILD=$(git cat-file -p $TREE_CHILD | cut -f2 -d" " | grep blob | wc -l)
         if [ $NUMBER_OF_BLOB_CHILDREN_OF_TREE_CHILD -eq 2 ]; then
-            exit 0
+            return 0
         fi
     fi
 done
 
-exit 1
+return 1