Merge branch 'jk/submodule-c-credential'

Merge branch 'jk/submodule-c-credential'

An earlier addition of "sanitize_submodule_env" with 14111fc4 (git:
submodule honor -c credential.* from command line, 2016-02-29)
turned out to be a convoluted no-op; implement what it wanted to do
correctly, and stop filtering settings given via "git -c var=val".

* 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