Fix problem with submodules on Windows
Fix problem with submodules on Windows

On Windows, `repo.create_submodule(...)` failed because git didn't recognize
the worktree path set in `.git/modules/sub/config` (`fatal: bad config file
line 6 in ./config`). This commit makes `_write_git_file_and_module_config`
convert the worktree path to the linux format (forward slashes) which git
recognizes.

expobrain/GitPython,manasdk/GitPython,breavo/GitPython,breavo/GitPython,StyXman/GitPython,StyXman/GitPython,pengshp/GitPython,ghtdak/GitPython,roidelapluie/GitPython,bwrsandman/GitPython,bwrsandman/GitPython,gitpython-developers/gitpython,StyXman/GitPython,jeblair/GitPython,roidelapluie/GitPython,pengshp/GitPython,dpursehouse/GitPython,expobrain/GitPython,manasdk/GitPython,avinassh/GitPython,zsjohny/GitPython,ghtdak/GitPython,dpursehouse/GitPython,beni55/GitPython,cdht/GitPython,pengshp/GitPython,jeblair/GitPython,gitpython-developers/gitpython,zsjohny/GitPython,zsjohny/GitPython,beni55/GitPython,cdht/GitPython,avinassh/GitPython,cdht/GitPython,nvie/GitPython,gitpython-developers/GitPython,gitpython-developers/GitPython,bwrsandman/GitPython,nvie/GitPython,avinassh/GitPython,h4ck3rm1k3/GitPython,h4ck3rm1k3/GitPython