Harden `git describe` parsing so it does not fail on awkward situations.

This issue came up because a user is using custom tags and git describe for him is something like `debian/0.11.1+ds-1-3-ga0afcb`.
Added unittests for this.
Also include the number of commits after the found tag.
git describe sha signatures have a length of 8 chars.
Merge pull request #3326 from s0undt3ch/hotfix/git-describe-parsing

Harden `git describe` parsing so it does not fail on awkward situations.