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! :)