GitCommandError: git merge --ff-only imports/c8/bind-9...

[sentry]

Sentry Issue: DIST2SRC-UPDATE-1N

Should have been fixed with #162 but regressed.

GitCommandError: Cmd('git') failed due to: exit code(1)
  cmdline: git merge --ff-only imports/c8/bind-9.11.20-5.el8_3.1
  stdout: 'Updating fda3718..6c9b251'
  stderr: 'error: Your local changes to the following files would be overwritten by merge:
	win32utils/build.txt
	win32utils/readme1st.txt
Please commit your changes or stash them before you merge.
Aborting'
(4 additional frame(s) were not displayed)
...
  File "dist2src/core.py", line 723, in update_source_git
    self.source_git.fast_forward(branch=dest_branch, to_ref=new_dest_branch)
  File "dist2src/core.py", line 223, in fast_forward
    self.repo.git.merge(to_ref, ff_only=True)
  File "git/cmd.py", line 542, in <lambda>
    return lambda *args, **kwargs: self._call_process(name, *args, **kwargs)
  File "git/cmd.py", line 1005, in _call_process
    return self.execute(call, **exec_kwargs)
  File "git/cmd.py", line 822, in execute
    raise GitCommandError(command, status, stderr_value, stdout_value)