Allow multiple level sets, pull out default order into "sequence" file

levels/bottom-up/index-add/congrats

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+There's another way to add an entry to the index directly:
+
+    git update-index --add --cacheinfo <mode>,<blobhash>,<name>
+
+The first three numbers of the mode describe the type of the entry, "100" is a regular file.
+
+The second three number describe the permissions. Only "644" (non-executable) and "755" (executable) are supported.
+
+You can insert the hash of an object into the terminal by right-clicking on it! :)

levels/bottom-up/index-add/description

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+Blobs usually represent the content of a file. But on their own, they don't have any metadata, not even a name!
+
+Git has a very powerful concept to store metadata related to blobs: the index! It's a list that relates blobs to filenames and access permissions.
+
+The most convenient option to add an entry to the index is via an existing file:
+
+    echo "my content" > file
+    git update-index --add file
+
+Add three entries to the index! For a bonus challenge: can you add a file that is inside of a directory, like "directory/file"?

levels/bottom-up/index-add/goal

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+echo "file 1" > file1
+echo "file 2" > file2
+echo "file 3" > file3
+git add .

levels/bottom-up/index-add/win

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+test "$(git ls-files | wc -l)" -ge 3