[utils/get_modified_files.py] fails with a few PR checkout tools
I have noticed that when using [gh cli](https://github.com/cli/cli) to checkout a pr
```
git merge-base --fork-point master
```
fails, which breaks `utils/get_modified_files.py`
e.g.:

```
gh pr checkout 9423
python utils/get_modified_files.py
Traceback (most recent call last):
  File "utils/get_modified_files.py", line 27, in <module>
    fork_point_sha = subprocess.check_output("git merge-base --fork-point master".split()).decode("utf-8")
  File "/home/stas/anaconda3/envs/main-38/lib/python3.8/subprocess.py", line 411, in check_output
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
  File "/home/stas/anaconda3/envs/main-38/lib/python3.8/subprocess.py", line 512, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['git', 'merge-base', '--fork-point', 'master']' returned non-zero exit status 1.
```

So `make fixup` fails to check the modified files then.

It fails if I use `git-pr` too (this tool is from the git-extras package).

It works fine if I use the native `git pr`.

This needs to be investigated.

Until this is resolved if you use those tools please use `make style` /  `make quality`, but `make fixup`  should work just fine elsewhere.
