GitRepository::currentBranch fails when on a detached HEAD

When in a detached HEAD state, e.g. by checking out a previous commit (e.g. git checkout HEAD~).

When running `git webrev -r HEAD~ -N` again we see the following exception:

$ git webrev -r HEAD~ -N
Exception in thread "main" java.io.IOException: Unexpected exit code
'git symbolic-ref --short HEAD' exited with status: 128
[stdout]
[stderr]
> fatal: ref HEAD is not a symbolic ref
        at org.openjdk.skara.vcs/org.openjdk.skara.vcs.git.GitRepository.await(GitRepository.java:94)
        at org.openjdk.skara.vcs/org.openjdk.skara.vcs.git.GitRepository.currentBranch(GitRepository.java:666)
        at org.openjdk.skara.cli/org.openjdk.skara.cli.GitWebrev.main(GitWebrev.java:198)
        at org.openjdk.skara.cli/org.openjdk.skara.cli.GitSkara.main(GitSkara.java:130)