/usr/share/doc/libsage-dev/changelog.Debian.gz is in libsage-dev 0.2.0-4.1ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/libsage-dev/../libsage2/changelog.Debian.gz
/usr/share/doc/libsage-dev/../libsage2/changelog.Debian.gz