Consider guarding against GitHub Pull Requests extension

[jennybc]

The GitHub Pull Requests extension does some funny business with branches that it perceives to be PR-related. I haven't delved into the details re: whether it's temporarily changing branch names or shimming some git command or some other third thing.

But whatever malarkey it is, it causes pr_finish() to fail like so:

✔ Switching back to default branch ("main").
✔ Pulling changes from "origin/main".
✔ Deleting local "switch-r-pkg-testing-shortcut" branch.
Error in libgit2::git_branch_delete : 
  could not find key 'branch.switch-r-pkg-testing-shortcut.github-pr-owner-number' to delete

What the heck is this?

branch.switch-r-pkg-testing-shortcut.github-pr-owner-number
                                    ^^^^^^^^^^^^^^^^^^^^^^^

The extension seems to assume that it will be mediating all of your git(hub) work and it can do something like this, which then breaks other tools.

Not sure if it's my / usethis's problem to solve but it's conceivable it would be worth detecting that suffix and trimming it off if this problem persists.