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>