checkout_pull_branch() fails when existing checked-out branch has local changes

[colinodell]

checkout_pull_branch() to perform a pull, but this doesn't work when the local copy has untracked changes:

Traceback (most recent call last):
  File "/pyenv/versions/[redacted]/lib/python3.9/site-packages/pygitops/operations.py", line 193, in get_updated_repo
    _checkout_pull_branch(repo, branch)
  File "/pyenv/versions/[redacted]/lib/python3.9/site-packages/pygitops/_util.py", line 82, in checkout_pull_branch
    origin.pull(branch)
  File "/pyenv/versions/[redacted]/lib/python3.9/site-packages/git/remote.py", line 910, in pull
    res = self._get_fetch_info_from_stderr(proc, progress,
  File "/pyenv/versions/[redacted]/lib/python3.9/site-packages/git/remote.py", line 750, in _get_fetch_info_from_stderr
    proc.wait(stderr=stderr_text)
  File "/pyenv/versions/[redacted]/lib/python3.9/site-packages/git/cmd.py", line 502, in wait
    raise GitCommandError(remove_password_if_present(self.args), status, errstr)
git.exc.GitCommandError: Cmd('git') failed due to: exit code(1)
  cmdline: git pull -v origin main
  stderr: 'error: Your local changes to the following files would be overwritten by merge:'

Suggested Fix

We could allow an optional force arg to be provided. If set and True, we would pass this option to the git checkout command to discard any changes to the index and working tree. An additional git clean command could also be run to remove untracked files.