Ref T10920, "assigning a diff to a commit post-facto" claimed that "This can be done via the CLI: ./bin/differential attach-commit" but I found when I tried that this wasn't the case.

Although the revision was updated with the commit, it was not actually associated with an edge, so when pushed, the revision wasn't recognised as related to the revision in associated tasks (we do not use arc land).

This diff adjusts attach-commit so it adds the edge, which I believe is what it is expected to do.

• Created a task
• Created a diff and associated it to the task
• Rebased the commit but did not arc diff to update the diff
• Pushed the commit
• Used bin/differential attach-commit to attach the commit to the diff
• Navigated to the task and the commit was correctly shown with the Differential revision beside it in the Commits section (Dxxx / rFOOBAR) rather than having the commit and revision listed separately