Fails on startup if repo is on a detached HEAD

[tommyclark]

File "/Users/tommy.clark/repos/asciigit/src/main.py", line 28, in main
    Scene([views.BranchListView(screen, branch_model)], -1, name="Main"),
  File "/Users/tommy.clark/repos/asciigit/src/views.py", line 85, in __init__
    model.list_branches(),
  File "/Users/tommy.clark/repos/asciigit/src/models.py", line 41, in list_branches
    if self.is_current_branch(branch):
  File "/Users/tommy.clark/repos/asciigit/src/models.py", line 52, in is_current_branch
    return branch == self.get_current_branch()
  File "/Users/tommy.clark/repos/asciigit/src/models.py", line 55, in get_current_branch
    return self.repo.active_branch
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/git/repo/base.py", line 696, in active_branch
    return self.head.reference
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/git/refs/symbolic.py", line 275, in _get_reference
    raise TypeError("%s is a detached symbolic reference as it points to %r" % (self, sha))
TypeError: HEAD is a detached symbolic reference as it points to '0ffadb42a0c95e4420ce9230fecdee32c70b7659'