Merge branch 'ps/config-global-override'

Merge branch 'ps/config-global-override'

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:
  t1300: fix unset of GIT_CONFIG_NOSYSTEM leaking into subsequent tests
  config: allow overriding of global and system configuration
  config: unify code paths to get global config paths
  config: rename `git_etc_config()`