Getting "Ref HEAD cannot be resolved" when starting JBoss EAP pointing to a remote git repository

Issue

I've been trying out the feature which allows the configuration files to be stored in git, but it's not working the way I expect, and there's something missing. I get the following error on startup:

/ 18:39:08,209 INFO  [org.jboss.modules] (main) JBoss Modules version 1.8.6.Final-redhat-00001
java.lang.RuntimeException: WFLYSRV0268: Failed to pull the repository https://<linktosomegit>/<user>/<repository>.git
        at org.jboss.as.server.controller.git.GitRepository.<init>(GitRepository.java:106)
        at org.jboss.as.server.ServerEnvironment.<init>(ServerEnvironment.java:539)
        at org.jboss.as.server.Main.determineEnvironment(Main.java:388)
        at org.jboss.as.server.Main.main(Main.java:96)
        at org.jboss.modules.Module.run(Module.java:352)
        at org.jboss.modules.Module.run(Module.java:320)
        at org.jboss.modules.Main.main(Main.java:593)
Caused by: org.eclipse.jgit.api.errors.RefNotFoundException: Ref HEAD cannot be resolved
        at org.eclipse.jgit.api.CreateBranchCommand.getStartPointObjectId(CreateBranchCommand.java:279)
        at org.eclipse.jgit.api.CreateBranchCommand.call(CreateBranchCommand.java:132)
        at org.eclipse.jgit.api.CheckoutCommand.call(CheckoutCommand.java:225)
        at org.jboss.as.server.controller.git.GitRepository.checkoutToSelectedBranch(GitRepository.java:156)
        at org.jboss.as.server.controller.git.GitRepository.<init>(GitRepository.java:91)
        ... 6 more
18:39:09,174 FATAL [org.jboss.as.server] (main) WFLYSRV0239: Aborting with exit code 1