app: store the revision number in a data file.
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).
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