substvar for additional commits on top of a tagged object

[Guillermo Gonzalez]

Would be nice to have the option to build a version number similar to the output of git describe --always --tags, example:

$ git clone lp:~telegraf-devs/telegraf/+git/telegraf
$ git -C telegraf describe --always --tags
1.0.0-beta2-21-gc046232

1.0.0-beta2-21-gc046232 which is composed by: {latest-tag}-{addiotional-commits}-{git-commit}