"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