4874: Create review from existing files in repo


What version are you running?


What's the URL of the page this enhancement relates to, if any?


Describe the enhancement and the motivation for it.

There are three ways to create a review currently:
- upload files
- from a linked repo:
- upload diff
- select a commit

It would be nice to also be able to create a review by selecting files from a linked repository.

This is useful in situations like:
- A project has been completed as a proof of concept, and is being transitioned into a production thing so the whole project needs review
- Someone is performing an ad-hoc or on-demand review of the current state of a project (or parts of the project). The relevant changes they are reviewing are split across many different commits (and partial commits) - the appropriate filter is files/directories, not commits.
- Design documentation is captured in a repository, rather than a wiki page - you don't want to delay improvements to the documentation by requiring review, but you want to support review of the design by others. (akin to Confluence's inline comments feature)

You can currently work around this by uploading the files from a local copy, but it would be easier and more reliable if you could select the files (+directories) from a revision.

It would be even better if the review could be updated with new commits, but I suspect that might get very complex given that it didn't start as a diff?