I realized having the revision as a build number is the wrong idea as it
implies packagers will have to rebuild GIMP for just a revision. Yet
very often revision may just be data change or dependency fix/update
without rebuild needed (i.e. no ABI change).
Instead let's keep this package information as a file 'gimp-release'
(inspired by /etc/os-release and other /etc/*-release files of
distributions).

(cherry picked from commit 87e9ebcf)