addition of first-changed-commits is not backwards-compatible

[tamird]

In a repo where git-filter-repo has already run, upgrading past d176bfb explodes:

Traceback (most recent call last):
  File "/Users/tamird/.local/bin/b4", line 8, in <module>
    sys.exit(cmd())
  File "/Users/tamird/.local/share/uv/tools/b4/lib/python3.8/site-packages/b4/command.py", line 417, in cmd
    cmdargs.func(cmdargs)
  File "/Users/tamird/.local/share/uv/tools/b4/lib/python3.8/site-packages/b4/command.py", line 83, in cmd_prep
    b4.ez.cmd_prep(cmdargs)
  File "/Users/tamird/.local/share/uv/tools/b4/lib/python3.8/site-packages/b4/ez.py", line 2831, in cmd_prep
    return edit_cover()
  File "/Users/tamird/.local/share/uv/tools/b4/lib/python3.8/site-packages/b4/ez.py", line 790, in edit_cover
    store_cover(new_cover, tracking)
  File "/Users/tamird/.local/share/uv/tools/b4/lib/python3.8/site-packages/b4/ez.py", line 659, in store_cover
    frf.run()
  File "/Users/tamird/.local/share/uv/tools/b4/lib/python3.8/site-packages/git_filter_repo.py", line 4936, in run
    self._record_metadata(self.results_tmp_dir(), self._orig_refs)
  File "/Users/tamird/.local/share/uv/tools/b4/lib/python3.8/site-packages/git_filter_repo.py", line 4767, in _record_metadata
    self._compute_metadata(metadata_dir, orig_refs)
  File "/Users/tamird/.local/share/uv/tools/b4/lib/python3.8/site-packages/git_filter_repo.py", line 4648, in _compute_metadata
    with open(os.path.join(metadata_dir, b'first-changed-commits'), 'br') as f:
FileNotFoundError: [Errno 2] No such file or directory: b'.git/filter-repo/first-changed-commits'

I believe that's because the new code assumes that the presence of already_ran implies the presence of first-changed-commits. This assumption is violated in the case of a version upgrade.