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