This is useful when the source is held in a git repository but this
repository is not an usual kwant repository.

Thanks to Bas Nijholt for reporting this.

* Only extract the version from Git when the top level directory of the Git
  repository coincides with the root directory of the Kwant distribution

* Disregard tags on branches that were merged-in (with Git >= 1.8.4).

The code should be easier to understand.  Configuration through a site.cfg file
is supported.