Added the git-describe implementation

This patch adds the implementation of git-describe(1).

I developed this for the Jenkins plugin to bring it to the parity with C Git implementation. This is my first patch to JGit, I think, so I expect there to be various issues. I appreciate your patience with me.

The command doesn't implement all the options available in C Git, but I think it solves the hard/tedious part that it's worthy of addition.

As explained inside the code comment, the performance can be improved if some additional hooks are exposed from RevWalk, namely `AbstractRevQueue.everybodyHasFlag` --- it allows me to cut a search quickly.

I hope you'll find this useful.