get_cwd_relative(): do not misinterpret root path

get_cwd_relative(): do not misinterpret root path

Commit 490544b (get_cwd_relative(): do not misinterpret suffix as
subdirectory) handles case where:

dir = "/path/work";
cwd = "/path/work-xyz";

When it comes to the end of get_cwd_relative(), dir is at '\0' and cwd
is at '-'. The rest of cwd, "-xyz", clearly cannot be the relative
path from dir to cwd. However there is another case where:

dir = "/";          /* or even "c:/" */
cwd = "/path/to/here";

In this special case, while *cwd == 'p', which is not a path
separator, the rest of cwd, "path/to/here", can be returned as a
relative path from dir to cwd.

Handle this case and make t1509 pass again.

Reported-by: Albert Strasheim <fullung@gmail.com>
Reported-by: Matthijs Kooijman <matthijs@stdin.nl>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>