Merge branch 'jl/maint-submodule-relative'

By Jens Lehmann (3) and Johannes Sixt (1)
* jl/maint-submodule-relative:
  submodules: fix ambiguous absolute paths under Windows
  submodules: refactor computation of relative gitdir path
  submodules: always use a relative path from gitdir to work tree
  submodules: always use a relative path to gitdir