make revision is now a target handled by cmake and depended on by mscore

make revision is now a target handled by cmake, which is depended on by the mscore target and always run when mscore is built. This means that if I 'git pull' and then 'make' in my build directory, the resulting build will have the correct revision. Likewise for if I do the above in Qt Creator.
The code also changes the revision to 'unknown' if it can't find Git -- according to the README in git/hooks, the repo is meant to have revision.h contain 'unknown' anyway, but it looks like an actual revision was committed by mistake last year. As the build script now generates revision.h automatically and in all cases, there is no need for revision.h to be in the repository at all, which should reduce the chance of it being accidentally committed in future.
The only user-facing change is that 'make revision' in the project directory no longer works, you have to call it in your build directory (though you should honestly never have to call it manually at all).