This function and normalize_absolute_path() do almost the same thing. The
former already works on Windows, but the latter crashes.

In subsequent changes we will remove normalize_absolute_path(). Here we
make the replacement function reusable. On the way we rename it to reflect
that it does some path normalization. Apart from that this is only moving
around code.
Signed-off-by: NJohannes Sixt <j6t@kdbg.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>