git version strings

This patch adds a function for converting a version string obtained from
`git describe` into a Buzzy version string.
Default value for `version` in git repositories

If the repositories that defines a source package is a clone of a git
repository, we now add a default value for the `version` variable which
parses the output of `git describe`, on the assumption that you're
creating tag names that include the latest released version number.
(The standard `git describe` output for describing commits after a
release tag produce Buzzy `+gitXXXXXXX` postrelease versions.)