If you missed something, there is already arc todo to create a task. But if you thing: I missed something at my diff, there is now way to plan changes after you submited the diff. So it would be very useful to let users do this in arcanist, this solved the problem to open phabricator, search the revision, and then clock changes, makes the whole process faster.
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That's before you're uploading the diff. If you already uploaded the diff, and say: "Oh, I forget to insert a parameter" for example, you can't use that.
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arc branch and arc list to see open revisions; arc browse D1234 to open it in browser; T5873 for everything else.
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I think this option would very rarely see use, and @avivey's suggestion (arc browse D1234 + use web UI) is a pretty good workaround.
Generally, I don't want to add arc commands unless they're pretty valuable (used pretty often, save a lot of time, etc). Adding rarely-used commands makes arc help overwhelming and makes it more difficult for new users to find the commands they need.
It's definitely not a goal to let arc do everything the web UI can do. In an extreme case, imagine how complicated arc edit-herald-rule would need to be.
After T5873 (specifically, once EditEngine comes to Differential) it will be possible to take this action over Conduit (it's possible that differential.createcomment can do it today via the action parameter), so you could use arc alias to define an arc plan-changes in terms of arc call-conduit.
After T5055 / T10329 it will be possible to more reasonably write and distribute this as an extension. You could then contribute it yourself, or convince someone else to maintain it.
The revision URI should also generally be printed to the console. My workflow in this case (where I want to add a comment, inline comment, or make some other followup metadata change) is to Control + Click the URL, then select "Open URL". Your terminal may have similar support. This is faster than searching for the revision.