The current behaviour of prefix_path is to return an empty string if
prefixing and absolute path that only contains exactly the work tree.
This behaviour is a potential regression point.
Signed-off-by: NMartin Erik Werner <martinerikwerner@gmail.com>
Reviewed-by: NDuy Nguyen <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>