I was setting up a buildbot for a Java project and I wanted to have a storing every built jar file, and I wanted to use git describe, so I ended up writing an ant plugin that emulates git-describe using jgit.
Download a copy of the jgit source and drop it into the jgit-describe folder. It should be called jgit-describe/jgit. Run ant. A jar file should appear in subfolder named dist.
Add a taskdef like the following to your build.xml.
Use the new git-describe task to populate a property with a string describing the current HEAD revision. dir is the path to the .git directory. property is the name of the property to populate.
jgit-describe.jar includes its own copy of the necessary parts of jgit.