CONFIG: refine handling of git information for debian builds
- When OpenFOAM is under git control and a 'debian/' directory exists, this could mean two things: 1) Additional debian control has been added to OpenFOAM 2) OpenFOAM has been imported into a debian project For the case that OpenFOAM has been imported into a debian project, using the git information would be highly misleading. There will be no OpenFOAM SHA1 correspondence. However, if additional debian control has been added to OpenFOAM the SHA1 will be valid. The ad hoc solution is to use an additional "openfoam.debian" directory to flag the addition of debian controls into openfoam. When a "debian/" directory exists without a "openfoam.debian", assume that the OpenFOAM has been imported into debian and do not use the SHA1.