Use the actual git branch and commit id

I have added code to extract the actual Git branch and commit id during the build process. As far as I can see this has been pretty much broken since the move from Subversion to Git.
I have also dropped the 'rev' prefix from the build artifacts as it seems entirely superfluous to me.

This all seems like a good idea to me!

I have enabled you to override this just as you could before, by setting the `git.commit` property at runtime. e.g. if you wanted to override it:
```bash
./build.sh -Dgit.commit=my-custom-id
```