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Produce contents of index with existing directory as cache

Is there a variant of “git checkout-index” which will produce the
existing index contents, like “git checkout-index” would do with an
empty directory, but can reuse an existing directory tree, to avoid
writing all files from scratch?

I'm writing some analysis scripts which need to change a checked-out
working tree.  Right now I'm throwing away the tree after making the
changes and create the next tree (which is expected to be quite similar
in contents) from scratch, starting with an empty directory.  This is
quite slow.