support --first-parent option in the jgit implementation of "git describe" #224

the current jgit based implementation doesn't have an option to choose tags by following the --first-parent. This is supposed to allow you to skip tags which are introduced by merges into the current branch. think multi-branch repositories where master/trunk represents "production". if you merge a hotfix forward from "master" into one of your develop branches, the annotated tag from the hotfix will be picked up instead of the tag on the development line.