persistent-https, url insteadof, and `git submodule`

[Elliott Cable <me@xxxxxx>]

Set up `persistent-https` as described in the [README][]; including the
‘rewrite https urls’ feature in `.gitconfig`:

    [url "persistent-https"]
        insteadof = https
    [url "persistent-http"]
        insteadof = http

Unfortunately, this breaks `git submodule add`:

    > git submodule add https://github.com/nodenv/nodenv.git \
        ./Vendor/nodenv
    Cloning into '/Users/ec/Library/System Repo/Vendor/nodenv'...
    fatal: transport 'persistent-https' not allowed
    fatal: clone of 'https://github.com/nodenv/nodenv.git' into
submodule path '/Users/ec/Library/System Repo/Vendor/nodenv' failed

Presumably this isn't intended behaviour?

   [README]: <https://github.com/git/git/tree/master/contrib/persistent-https#readme>


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