savewd: wrong behavior after recent commit

[Grisha Levit]

The coreutils test mkdir/p-3.sh currently fails on macOS, AFAICT as a
result of the gnulib change:

    e4399abd41 (savewd: port to native MS-Windows, 2024-08-27)

The relevant part of the test is:

    mkdir no-access || framework_failure_

    p=$(pwd)
    (cd no-access && chmod 0 . && mkdir -p "$p/a/b" u/v) 2> /dev/null
    test $? -eq 1 || fail=1

Essentially, if cwd lacks search permission, relative paths in the list
passed to savewd_process_files end up resolved relative to the dir used
to process any previous absolute path in the list.