Merge branch 'jk/submodule-c-credential'

Merge branch 'jk/submodule-c-credential'

"git -c credential.<var>=<value> submodule" can now be used to
propagate configuration variables related to credential helper
down to the submodules.

* jk/submodule-c-credential:
  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