refactoring + small added functionality (did not test as ANT task)

You could view this mostly as a 'refactoring' of your code.  I didn't want the ANT task part as much as I just wanted the 'git-describe' code.  So I...
* separated it into another class as a "builder" style
* added the dirty information (e.g. like git describe --dirty)
* found a simpler way of dealing with instantiating the Repository using setWorkTree().  But I might have got it wrong.  I'm a JGit newbie.
* reconnected your ANT task code to the new class.  **WARNING: i did not test it as an ANT task yet**.  Maybe you want to test that and if you like pull the changes.