implemented is-ancestor for git 1.8+

Deciding whether a commit precedes another  not easy due to the non-linear development in Git. Dates are not reliable anymore, nor is the order they appear in the log. This PR makes used of 'git merge-base --is-ancestor` introduced in Git 1.8 to decide that. I am not sure whether we should support earlier versions of Git.

@andygrunwald, @sduenas, comments?