Allow glab mr checkout to handle non fast-forward conflicts
### Problems to solve
- `glab mr checkout XXXXX` fails if your local copy of the branch is behind the remote, and a fast-forward isn't possible.
- You can't `git pull` (or similar) on a branch created by `glab mr checkout` since no upstream tracking branch is set up for it.
### Proposal
- Offer to rebase your local branch onto upstream's if a fast-forward isn't possible.
- Offer to set up the remote tracking branch after running `glab mr checkout`.
- For MRs from forks, first offer to set up the remote to point to the fork repo.
Bonus points: for both of these, make the default answer configurable in `glab`'s settings.
### Links / references
issue