Embed the git revision in the doxygen documentation.
The simple git-version script asks git for the current revision, and falls back to 'unknown' if the result is empty. This will have little effect until we have tags in the repository. We use the FILE_VERSION_FILTER option on doxygen to embed this in the documentation with each file reference. Unfortunately, there doesn't seem to be a way to set PROJECT_NUMBER globally in the same way. This commit unsets that option to avoid confusing references in the generated documentation.