Enhancements glab mr checkout

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