Merge branch 'sb/submodule-helper'

The infrastructure to rewrite "git submodule" in C is being built
incrementally.  Let's polish these early parts well enough and make
them graduate to 'next' and 'master', so that the more involved
follow-up can start cooking on a solid ground.

* sb/submodule-helper:
submodule: rewrite `module_clone` shell function in C
submodule: rewrite `module_name` shell function in C
submodule: rewrite `module_list` shell function in C

diff --cc Makefile
Simple merge

diff --cc builtin.h
Simple merge

diff --cc git-submodule.sh
Simple merge

diff --cc git.c
Simple merge