t0060: split absolute path test in two to exercise some of it on Windows

t0060: split absolute path test in two to exercise some of it on Windows

Only the first half of the test works only on POSIX, the second half
passes on Windows as well.

A later test "real path removes other extra slashes" looks very similar,
but it does not make sense to split it in the same way: When two slashes
are prepended in front of an absolute DOS-style path on Windows, the
meaning of the path is changed (//server/share style), so that the test
cannot pass on Windows.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Acked-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>