make_absolute_path(): Do not append redundant slash

make_absolute_path(): Do not append redundant slash

When concatenating two paths, if the first one already have '/', do
not put another '/' in between the two paths.

Usually this is not the case as getcwd() won't return '/foo/bar/',
except when you are standing at root, then it will return '/'.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>