pr_finish(n) dies in pr_fetch() if remote branch already deleted

[jennybc]

 Checking out PR 'r-lib/gh/#121' (@jennybc): 'Accept (and prioritize) URI templates (a la RFC 6570)'
✓ Switching to branch 'curly-bracket-uri-template'
✓ Setting 'origin/curly-bracket-uri-template' as remote tracking branch
Error in libgit2::git_branch_set_upstream : 
  cannot set upstream for branch 'curly-bracket-uri-template'

As a result, we never get to all the cleanup code, e.g. deleting the local branch.

Current workaround: just call pr_finish() (no argument!) again.

[jennybc]

I don't see an easy fix, i.e. it suggests some nontrivial refactoring where we proactively identify the PR situation. Relevant facts:

• Is the PR internal to the source repo or is it external (coming from a fork)?
• Current user's role/permission, e.g. internal or external contributor
• Whether the PR has ever been checked out locally before

Instead of having one logical flow that handles all scenarios, it might be easier to characterize the situation and use logic specific to that scenario. This comment is nominally about pr_finish() but may also apply to pr_fetch().