This is a minor refactor of how `_warn_use_shell` can be, and is,invoked.The `_warn_use_shell` helper function in `git.cmd` takes a single`bool`-valued argument `extra_danger`, which is conceptuallyassociated with having a `True` value of `USE_SHELL`, but theassociation is not necessarily obvious. Specifically:- For the warning given when reading `USE_SHELL` on the `Git` class  or through an instance, `extra_danger` is always `False`. This is  so even if the `USE_SHELL` value is currently `True`, because the  danger that arises from `True` occurs internally.- For the warning given when writing `USE_SHELL`, which can only be  done on the `Git` class and not on or through an instance,  `extra_danger` is the value set for the attribute. This is  because setting `USE_SHELL` to `True` incurs the danger described  ingitpython-developers#1896.When reading the code, which passed `extra_danger` positionally,the meaning of the parameter may not always have been obvious.This makes the `extra_danger` parameter keyword-only, and passesit by keyword in all invocations, so that its meaning is clearer.