gh stack modify fails with untracked files

`gh stack modify` preconditions fail if the current working directory has any untracked files and fails with the following message that seems to imply that untracked changes shouldn't be a problem, as there is nothing to stash or commit.

```
✗ uncommitted changes in working tree
Commit or stash your changes before running modify
```

Code that performs the check: https://github.com/github/gh-stack/blob/5ed5693ed6fcfa34228312abb09553e2580b6670/internal/git/gitops.go#L576

Perhaps this should ignore any lines starting with `?? `?