Add support for reading issue field values - #14130

The other half of #99. The library could answer the question; nothing could
supply it with a real change. Now roz codeowners --pr does the whole thing in
one command.

github.Change fetches three things: the paths a pull request touches, the
CODEOWNERS governing them, and who has already approved. The approvals matter
as much as the files -- the useful question is what is left, and having to look
that up separately and type it back in is most of the friction.

CODEOWNERS is read from the base ref, not the default branch, because a change
into a release branch is governed by that branch's rules. That is why it is two
round trips: the base ref is not known until the pull request has been read.
The three locations GitHub looks in are tried as three aliases in one query,
since a repository has at most one of them.

The file list is paginated to completion. A partial list could turn "no single
owner covers this" into "one does", and being wrong in that direction is worse
than being slow.

Deliberately not part of the batched poll, which is what #99 assumed and is
right for a different reason than it gives: the cost is not that it cannot be
batched, it is that the file list is large, changes only when someone pushes,
and is wanted when a person asks a question rather than every minute.

Verified against cli/cli#14130: sixteen files, rules read from
.github/CODEOWNERS@trunk, one team covering all of it.

Say plainly when no rule matches any file, rather than reporting that no
single owner covers everything. Found running this against elasticsearch,
whose CODEOWNERS covers eighty specific paths in a very large repository, so
most changes match nothing: the old wording read as a problem and sat beside a
contradictory "nothing outstanding".

Matches tuicr, which takes the pull request as a subcommand argument
rather than a flag -- it is the object you are acting on, not a modifier
on some other action.

`<target>` accepts the three forms people actually have in hand:

  diffident pr 123
  diffident pr cli/cli#13758
  diffident pr cli/cli#13758

The last two carry their own repo, which is the point: pasting a URL
should not also require saying which repository it came from. URLs still
parse with whatever GitHub appends as you click around -- /files,
#discussion_r123, ?w=1. A bare number uses the checkout you are in.

`--repo` still overrides, because a flag is a statement where a target's
repo is only an inference. `--repo`/`--pr` keep working unchanged.

473 tests, clippy clean. All three forms verified against real pull
requests: cli/cli#13758 by URL, cli/cli#14130 by slug, and #89 by bare
number in this checkout.