Rework levels, and add some new ones

levels/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 permissions.
+
+Even though it is possible to add an entry directly to the index, it's much more convenient to do it 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/index-add/goal

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

levels/index-add/win

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