The idea is to be able to advertize a new revision of the same version
of GIMP. For instance, this would apply when we release a
`gimp-2-10-14-setup-3.exe` Windows installer (then we are at revision
3, provided we started at revision 0).

The revision number is obviously only relevant to a given platform and
version. Also the concept of build ID allows to differentiate various
builds for a same platform, in particular to not look at revisions of
third-party builds. The build ID can be any string. Maybe we could just
use reverse DNS build id (such as "org.gimp.GIMP_official") to identify
the official GIMP build. So in the end, we only compare revisions for an
identical (version, platform, build-id) tuple.