git-ubuntu support for worktrees

[Sergio Durigan Junior]

I like using worktrees to work on more complex packages, and I've just noticed that "git ubuntu prepare-upload args" doesn't work when inside one.

This is what I see:

 --git-ubuntu-prepare-upload-args-failed
 Traceback (most recent call last):
   File "/snap/git-ubuntu/1332/usr/bin/git-ubuntu", line 11, in <module>
     load_entry_point('gitubuntu==1.1', 'console_scripts', 'git-ubuntu')()
   File "/snap/git-ubuntu/1332/usr/lib/python3/dist-packages/gitubuntu/__main__.py", line 270, in main
    sys.exit(args.func(args))
   File "/snap/git-ubuntu/1332/usr/lib/python3/dist-packages/gitubuntu/prepare_upload.py", line 478, in cli_printargs
     repo = gitubuntu.git_repository.GitUbuntuRepository('.')
   File "/snap/git-ubuntu/1332/usr/lib/python3/dist-packages/gitubuntu/git_repository.py", line 1113, in __init__
     self.raw_repo = pygit2.init_repository(self._local_dir)
   File "/snap/git-ubuntu/1332/usr/lib/python3/dist-packages/pygit2/__init__.py", line 149, in init_repository
     check_error(err)
   File "/snap/git-ubuntu/1332/usr/lib/python3/dist-packages/pygit2/errors.py", line 46, in check_error
     raise ValueError(message)
 ValueError: failed to make directory '/home/sergio/work/qemu/qemu-ibm-lp2003673/.git': directory exists

The problem happens because of how GitUbuntuRepository is initialized:

https://git.launchpad.net/git-ubuntu/tree/gitubuntu/git_repository.py#n1084

            try:
                os.mkdir(local_dir)
            except FileExistsError:
                local_dir_list = os.listdir(local_dir)
                if local_dir_list and os.getenv(
                    'GIT_DIR',
                    '.git',
                ) not in local_dir_list:
                    logging.error('Specified directory %s must either '
                                  'be empty or have been previously '
                                  'imported to.', local_dir)
                    sys.exit(1)

It assumes that ".git" will always be a directory, but inside worktrees ".git" is actually a regular file.