Merge branch 'jt/submodule-name-to-gitdir'

Code refactoring.

* jt/submodule-name-to-gitdir:
  submodule: extract path to submodule gitdir func

diff --cc builtin/submodule--helper.c
Simple merge

diff --cc dir.c
Simple merge

diff --cc submodule.c
Simple merge

diff --cc submodule.h
Simple merge