Show current config and source using arc set-config --show

Summary:
`arc set-config --show` only show the user config
It would be better to contain local/global/system config

Test Plan: set config by local/global/system/user/project, and check the result of `arc set-config --show`

Reviewers: #blessed_reviewers, epriestley

Reviewed By: epriestley

CC: epriestley, aran

Differential Revision: https://secure.phabricator.com/D7851