According to the arc diff user guide (https://secure.phabricator.com/book/phabricator/article/arcanist_diff/#git), the default commit range that is sent if you don't specify a commit is:

`git merge-base origin/master HEAD`..HEAD

But if you're on a branch that has an upstream it will instead be the merge-base of the upstream branch and your local branch (which makes sense, since that's "all the commits on the current branch that you haven't pushed yet", but the explicit origin/master in the previous sentence is confusing).