Merge branch 'sb/submodule-init'

Update of "git submodule" to move pieces of logic to C continues.

* sb/submodule-init:
  submodule init: redirect stdout to stderr
  submodule--helper update-clone: abort gracefully on missing .gitmodules
  submodule init: fail gracefully with a missing .gitmodules file
  submodule: port init from shell to C
  submodule: port resolve_relative_url from shell to C

Author: gitster