Create a patch by comparing a specific branch on the remote with a specific local branch
Solution 1:
Use git format-patch origin/master. This creates a patch file for each commit on your checked out branch, which is not in origin/master.
To have one file instead of multiple files you can use
git format-patch master --stdout > mypatch.patch
Solution 2:
Use this to create one file containing all commits in your current branch that are not in the master branch:
git format-patch master --stdout > mypatch.patch