Add a flag to prevent creating MRs while `stack sync`ing
More [good feedback](https://gitlab.com/gitlab-org/cli/-/issues/7473#note_1960595503) from @nathan.cain:
> Side note: I'd want either a flag like `glab stack sync --pull` or new keyword like `glab stack pull` to only pull and rebase changes as need, but not create a new mr for my currently under edit diff. This goes back to wanting to be explicit about my actions, in this case being explicit about when I submit work for review.
Also could consider:
* `glab stack --pull` (only pull new changes)
* Adding a warning for the above situation?
issue