Since majority of Differential users are using Arcanist to create new Differential Revisions it's obvious that there no such "Create Diff" option in "+" icon dropdown in header.
Unlucky for our company we are using "svn:externals" and Arcanist can't be used because if that. Therefore please add "Create Diff" (or similarly named) option to dropdown that appears when clicking on "+" icon (next to avatar) in header.
This is pretty easy to maintain as a local patch or by bookmarking it in the browser. We're not interested in adding more items currently to that menu and T5867 solves this in a better way.
Is there any instruction on how to create local patch and keep update_phabricator.sh script work without causing any conflicts with git pull? If I remember properly then git pull simply won't work when it detects local changes.
Forking, Local Patches, etc, are outside of the scope for what we can provide support for. I would recommend Stack Overflow or Google for learning patching/shell scripting.