/usr/share/doc/isc-dhcp-relay-dbg/changelog.Debian.gz is in isc-dhcp-relay-dbg 4.2.4-7ubuntu12.12.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/isc-dhcp-relay-dbg/../isc-dhcp-common/changelog.Debian.gz