Merge branch 'ps/config-env-pairs'

Merge branch 'ps/config-env-pairs'

Introduce two new ways to feed configuration variable-value pairs
via environment variables, and tweak the way GIT_CONFIG_PARAMETERS
encodes variable/value pairs to make it more robust.

* ps/config-env-pairs:
  config: allow specifying config entries via envvar pairs
  environment: make `getenv_safe()` a public function
  config: store "git -c" variables using more robust format
  config: parse more robust format in GIT_CONFIG_PARAMETERS
  config: extract function to parse config pairs
  quote: make sq_dequote_step() a public function
  config: add new way to pass config via `--config-env`
  git: add `--super-prefix` to usage string