Allow reviews of full stacks of pull requests

[sl]

I'm part of the beta, and it's been great using it so far. My only feedback though is: when someone else creates a stack of PRs, I want to provide review feedback across the whole stack, but the reviews are split entirely per-PR.

It would be great to be able to prepare a review across a full stack, and leave all comments at once. In particular, it would be most useful to:

• be able to mass approve a full stack
• in one sweep leave comments on everything in a stack, then at the end when I submit my review, select which parts of the stack to approve, and which to block

[CyanoFresh]

yes, wanna see a full picture

[Kache]

related, squashed diff view: #198

[BRaimbault]

+1. We split large features into stacks of small pull requests, so each part is easy to read on its own. The authors choose that split, but reviewers would also like the option to see everything together, to get the entire context. The docs make the same point: "Reviewing a single change out of context from the rest of the stack can reduce review quality."

Today, the stack map, the stack header and Shift+J / Shift+K make it easy to move between parts, but each part is still read, commented on and approved on its own. The compare view (base...top) shows the full diff, but it isn't a pull request: no review comments, no approvals, and no "Viewed" checkboxes to keep track of progress.

As a reviewer, I would like to:

1. read the whole stack as one diff, see which part each change comes from, and still switch to a single part when needed
2. mark files as viewed across the whole stack
3. comment across several parts in one pending review, and submit it once
4. on submit, approve the whole stack at once, or choose per part: approve, request changes or comment

It would work for us if submitting still recorded one review per pull request, so the branch protection and CODEOWNERS approvals that apply to every pull request in a stack keep working as they do today.

---

Similar need has come up elsewhere:

• add support to view all changes in stack #198: view the diffs of the whole stack in a single view
• View line changed in future branch #208: see whether a line in a lower part is changed by a later part
• Assigning the same reviewers to the whole stack #253: assign the same reviewers to the whole stack
• I wish there was a dedicated page to link to a stack #251: a dedicated page to link to a stack
• In the public preview feedback discussion: review the entire stack at once (1), review either the current pull request or the whole chain (2), see the whole stack diff in one go (3), request reviewers and leave comments for the stack as a whole (4), and see which parts of a stack you have already reviewed (5)