java.lang.IllegalArgumentException: One of setGitDir or setWorkTree must be called.

[frankgerhardt]

Debian 8, OpenJDK 8, fitnesse-standalone.jar, fitnesse-git-plugin-1.2.0-all.jar.

Fresh setup, no FitNesseRoot. I edit the TwoMinuteExample, works fine. Then I enable the git plugin as in the README.

$ java -jar fitnesse-standalone.jar -p 8080
Bootstrapping FitNesse, the fully integrated standalone wiki and acceptance testing framework.
root page: fitnesse.wiki.fs.FileSystemPage at ./FitNesseRoot#latest
logger: none
authenticator: fitnesse.authentication.PromiscuousAuthenticator
page factory: fitnesse.html.template.PageFactory
page theme: bootstrap
Starting FitNesse on port: 8080
WARNING: An error occured while fulfilling user request [One of setGitDir or setWorkTree must be called.]
java.lang.IllegalArgumentException: One of setGitDir or setWorkTree must be called.
	at org.eclipse.jgit.lib.BaseRepositoryBuilder.requireGitDirOrWorkTree(BaseRepositoryBuilder.java:586)
	at org.eclipse.jgit.lib.BaseRepositoryBuilder.setup(BaseRepositoryBuilder.java:554)
	at org.eclipse.jgit.storage.file.FileRepositoryBuilder.build(FileRepositoryBuilder.java:92)
	at fitnesse.wiki.fs.GitFileVersionsController.getRepository(GitFileVersionsController.java:223)
	at fitnesse.wiki.fs.GitFileVersionsController.makeVersion(GitFileVersionsController.java:133)
	at fitnesse.wiki.fs.FileSystemPage.commit(FileSystemPage.java:132)
	at fitnesse.responders.editing.SaveResponder.saveEdits(SaveResponder.java:43)
	at fitnesse.responders.editing.SaveResponder.makeResponse(SaveResponder.java:29)
	at fitnesse.FitNesseExpediter.createGoodResponse(FitNesseExpediter.java:145)
	at fitnesse.FitNesseExpediter.makeResponse(FitNesseExpediter.java:112)
	at fitnesse.FitNesseExpediter.run(FitNesseExpediter.java:51)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.lang.Thread.run(Thread.java:834)

What's wrong? It's unclear to me if there should be a git repo already or not and who initializes that. Who is the git user?