Merge branch 'jk/submodule-c-credential' into js/http-custom-headers

Merge branch 'jk/submodule-c-credential' into js/http-custom-headers

* jk/submodule-c-credential:
  submodule: stop sanitizing config options
  submodule: use prepare_submodule_repo_env consistently
  submodule--helper: move config-sanitizing to submodule.c
  submodule: export sanitized GIT_CONFIG_PARAMETERS
  t5550: break submodule config test into multiple sub-tests
  t5550: fix typo in $HTTPD_URL
  git_config_push_parameter: handle empty GIT_CONFIG_PARAMETERS
  git: submodule honor -c credential.* from command line
  quote: implement sq_quotef()
  submodule: fix segmentation fault in submodule--helper clone
  submodule: fix submodule--helper clone usage
  submodule: check argc count for git submodule--helper clone
  submodule: don't pass empty string arguments to submodule--helper clone