Try the built-in version string before git-describe

Try to get the built-in version string first, and fall back to git
describe if there is no built-in string, instead of the other way
around. This makes computing the version string much cheaper in the
common case (whenever StGit is not run directly from a git-controlled
tree).

In order for this to work when StGit _is_ run directly from a
git-controlled tree, setup.py has to delete the builtin version file
once the installation process is over. (Otherwise, the StGit version
in a git-controlled tree would be frozen at whatever value it happened
to have when setup.py was last run.)

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