This commit expands on @nobe4 initial work by bringing the owner and repository name to the most obvious issue and PR number output use cases before making a second pass for any edge cases.
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.
