Pass `--global` when calling `git config --get`, for consistency with `git config...

Pass `--global` when calling `git config --get`, for consistency with `git config --set` (#23157) (#23198)

Backport #23157

This arose out of #22451; it seems we are checking using non-global
settings to see if a config value is set, in order to decide whether to
call another global(-indeed) configuration command. This PR changes it
so that both the check and the set are for global configuration.

Co-authored-by: Philip Peterson <philip-peterson@users.noreply.github.com>