Could you please edit "Actual results" by splitting it into several sentences with punctuation, so it is easier to read and understand for others? Thank you!

And you should be able to apply it against the current branch your on so if to git checkout wmf/stable it should be applied against that branch.

https://secure.phabricator.com/book/phabricator/article/arcanist_commit_ranges/

In Git and Mercurial, many arc commands (notably, arc diff) operate on a range of commits beginning with some commit you specify and ending with the working copy state.

Since the end of the range is fixed (the working copy state), you only need to specify the beginning of the range. This is called the "base commit". You can do this explicitly when you run commands:

$ arc diff HEAD^ # git: just the most recent commit

Oh, is there a way we can get it to do it behind the scenes like gerrit. Since it checkouts out the branch that has the revision.

And this is helpful in understanding what arc is doing:

http://stackoverflow.com/questions/19819252/can-you-arc-diff-against-a-particular-git-branch?rq=1