Do not error in race condition of directory existing

[AlanCoding]

This address a rare race condition where the directory is created by simultaneous processes both trying to create things under the same folder.

ansible/awx#6119

File "/awx_devel/awx/main/tasks.py", line 2255, in make_local_copy
  source_branch = git_repo.create_head(tmp_branch_name, scm_revision)
File "/venv/awx/lib/python3.6/site-packages/git/repo/base.py", line 389, in create_head
  return Head.create(self, path, commit, force, logmsg)
File "/venv/awx/lib/python3.6/site-packages/git/refs/symbolic.py", line 546, in create
  return cls._create(repo, path, cls._resolve_ref_on_create, reference, force, logmsg)
File "/venv/awx/lib/python3.6/site-packages/git/refs/symbolic.py", line 513, in _create
  ref.set_reference(target, logmsg)
File "/venv/awx/lib/python3.6/site-packages/git/refs/symbolic.py", line 329, in set_reference
  assure_directory_exists(fpath, is_file=True)
File "/venv/awx/lib/python3.6/site-packages/git/util.py", line 181, in assure_directory_exists
  os.makedirs(path)
File "/venv/awx/lib64/python3.6/os.py", line 220, in makedirs
  mkdir(name, mode)
FileExistsError: [Errno 17] File exists: '/var/lib/awx/projects/_71__project_aspectmatter/.git/refs/heads/awx_internal'

This has been available since python 3.2, and I see in your setup.py that you don't support python2 any longer, so this kwarg shouldn't cause problems.

[Byron]

Thanks so much! It's so painful to track these things down usually, and am glad it's as simple as adding another argument.