t0060: add test for prefix_path on symlinks via absolute paths

t0060: add test for prefix_path on symlinks via absolute paths

When symlinks in the working tree are manipulated using the absolute
path, git dereferences them, and tries to manipulate the link target
instead.

This applies to most high-level commands but prefix_path is the common
denominator for all of them.

Add a known-breakage tests using the prefix_path function, which
currently uses real_path, causing the dereference.

Signed-off-by: Martin Erik Werner <martinerikwerner@gmail.com>
Reviewed-by: Duy Nguyen <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>