/usr/share/doc/qtpurchasing5-dbg/changelog.Debian.gz is in qtpurchasing5-dbg 5.6.0~git20151023.2f454143-0ubuntu4.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/qtpurchasing5-dbg/../libqt5purchasing5/changelog.Debian.gz