When using Git, if I rebase my work on the current master, I need to specify `arc diff --update DXXXX` for it to understand that I want to update an existing review and not open a new one.
Arcanist could store in git's local config the id of the review and use that, say, run `git config --local --add branch.<current_branch>.review DXXXX` and check `git config --local branch.<current_branch>.review` to see if there is a review on that branch already, and use this when I run `arc diff`.