Better StGit version tracking

Instead of claiming to be the latest released version (really, a
hardcoded string that we hope is the latest released version), run git
describe to figure out what version we are, just like git does. Fall
back to a hardcoded value that is generated at install time, or
supplied in a release tarball.

Currently, we have to give git describe the --tags flag, since StGit
release tags are lightweight tags. This means we're going to pick up
any lightweight tags the user makes, which isn't ideal. The solution
is to start making annotated release tags, and then remove that flag.

Signed-off-by: Karl Hasselström <kha@treskal.com>