Fix `git svn rebase` if top-level HEAD directory exist

I have svn repo with top-level directory named HEAD and `git svn rebase` fails with 
```
$ git svn rebase HEAD --
fatal: ambiguous argument 'HEAD': both revision and filename
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
rev-list --first-parent --pretty=medium HEAD: command returned error: 128
```

Works fine with patch from pull request.