I am working on Emacs and hope to submit a patch. To create the patch, I need to do a diff between my revised code and master. How do I do this?
You can diff your working tree against the master like this:
bzr diff --new lp:emacs
If lp:emacs is registered as the parent branch, then this works too:
bzr diff --new :parent
But I really don't think you need this for submitting a patch. The diff will include not only the changes you did, but the changes that were added to master since you cloned from it. I think it will be difficult to merge this.
I'm not familiar with the development cycle of the Emacs project, but normally you would push your branch to your own Launchpad account and then create a merge proposal from it using the web interface.
Or, maybe you're looking for the send command instead:
bzr send -o my.patch :parent
This creates a my.patch file which you can send to the maintainers. It bundles inside all your local revisions that are not in master. The maintainers can easily merge this. It also contains a diff, but that's just for information, so you can read what's in the bundle.