Git submodule: transport 'keybase' not allowed

I can clone and push to Keybase repos, but cloning one as a submodule seems not to recognise the `keybase` scheme:
```
$git submodule add keybase://private/ojford/repo
Cloning into '$PWD/repo'...
fatal: transport 'keybase' not allowed
fatal: clone of 'keybase://private/ojford/repo' into submodule path '$PWD/repo' failed
$uname -or
18.6.0 Darwin
$keybase --version
keybase version 4.2.0-20190710181120+e7c0bdc4a2
$git --version
git version 2.22.0
```

Reproduce the reported `git submodule add keybase://private/ojford/repo` command and compare it with the direct Keybase clone and push that work. Start by tracing how Git handles allowed transports for submodules; done means the Keybase URL can be added as a submodule without the “transport 'keybase' not allowed” failure.

Written by the indexing model from the issue text.