submodules: fix ambiguous absolute paths under Windows

submodules: fix ambiguous absolute paths under Windows

Under Windows the "git rev-parse --git-dir" and "pwd" commands may return
either drive-letter-colon or POSIX style paths. This makes module_clone()
behave badly because it expects absolute paths to always start with a '/'.

Fix that by always converting the "c:/" notation into "/c/" when computing
the relative paths from gitdir to the submodule work tree and back.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>