Error on getCurrentBranch() If No Branch is Checked Out

[tsteward]

Steps to reproduce

1. Checkout a specific commit hash git checkout <commit hash>
2. Call getCurrentBranch() on a GitDir instance.
3. The following error occurs.

Error

RangeError: Value not in range: 11
#0      _StringBase.substring (dart:core/runtime/libstring_patch.dart:384:7)
#1      new BranchReference (package:git/src/branch_reference.dart:11:32)
#2      CommitReference.toBranchReference (package:git/src/commit_reference.dart:35:11)
#3      GitDir.getCurrentBranch (package:git/src/git_dir.dart:116:10)