How can i prepare a phabricator revision for a new repository?

So i usually have a feature branch and then arc diff master to create a differential revision off master branch.

How will i make a revision on a repository that has only the master branch?

Should i have started with an empty branch or commit to record the whole diffs? Is there a standard way to do this in phabricator?

Solution

You can use arc diff origin/master to generate a diff between the origin's master branch and your local master branch.
You can move your changes to a local branch, as though you had started on an empty branch. This is a general git workflow and you can search for instructions if you aren't sure how to do it.