Also retrieve tags for a cached git repo.

If someone refers to a tag in the revision field, git fetch might
not necessarily pulled that object.

As documented, the safe way is to run git fetch _and_
git fetch --tags
Merge pull request #39 from thorstenb/fix-git-fetch-tags

Also introduce some line-breaks to fix PEP8 errors.
add pep8 checking to GNUmakefile