Command 'check --has-new-runs' crashes in a detached commit.

[atimin]

Describe the bug
Command 'check' with flag '--has-new-runs' crashes if it is run in a detached commit

To Reproduce
Steps to reproduce the behavior:

1. In the project directory run git checkout 0.3.0
2. Run python -m nitpicker run --has-new-runs
3. See error:

-----------------------------------
Check if all the last runs passed.
-----------------------------------
Plancommands.add has 0 failed and 0 skipped of 2 test cases
Plancommands.check has 0 failed and 0 skipped of 2 test cases
Plancommands.run has 0 failed and 0 skipped of 1 test cases
Totally your project has 0 failed and 0 skipped of 5 test cases
-----------------------------------
Check if current branch has new runs.
-----------------------------------
Traceback (most recent call last):
  File "/opt/python/3.3.6/lib/python3.3/runpy.py", line 160, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/opt/python/3.3.6/lib/python3.3/runpy.py", line 73, in _run_code
    exec(code, run_globals)
  File "./nitpicker/__main__.py", line 3, in <module>
    nitpicker.main()
  File "/home/travis/virtualenv/python3.3.6/lib/python3.3/site-packages/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/home/travis/virtualenv/python3.3.6/lib/python3.3/site-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/home/travis/virtualenv/python3.3.6/lib/python3.3/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/travis/virtualenv/python3.3.6/lib/python3.3/site-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/travis/virtualenv/python3.3.6/lib/python3.3/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/home/travis/virtualenv/python3.3.6/lib/python3.3/site-packages/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "./nitpicker/nitpicker.py", line 138, in check
    success &= handler.check_has_new_runs()
  File "./nitpicker/commands/check_command_handler.py", line 79, in check_has_new_runs
    diffs = list(self.__cvs_adapter.diff(self.__main_branch))
  File "./nitpicker/cvs/git_adapter.py", line 28, in diff
    branch = self.__repo.active_branch
  File "/home/travis/virtualenv/python3.3.6/lib/python3.3/site-packages/git/repo/base.py", line 696, in active_branch
    return self.head.reference
  File "/home/travis/virtualenv/python3.3.6/lib/python3.3/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 '670365d9258c7fd89e4ba43160d78c1a5f698f49'

Expected behavior
The program should not crash