Extract the ad-hoc PAGER behavior from `pr diff` command and make it available opt-in to any command through IOStreams.
pr -R
If the `--repo` flag is specified, then the user intends to select a repository other than the current one. In that case, it doesn't make sense to fall back to detecting the PR belonging to the current branch, so throw a descriptive error instead.
--repo
pr
pr diff