Replace GIT_CONFIG_NOSYSTEM mechanism to decline from reading the
system-wide configuration file with GIT_CONFIG_SYSTEM that lets
users specify from which file to read the system-wide configuration
(setting it to an empty file would essentially be the same as
setting NOSYSTEM), and introduce GIT_CONFIG_GLOBAL to override the
per-user configuration in $HOME/.gitconfig.
* ps/config-global-override:
config: allow overriding of global and system configuration
config: unify code paths to get global config paths
config: rename `git_etc_config()`