When other guys had already given some answer which comply with git convention, the OP’s question, “create a patch without commit”, can be also solved in this way:
git diff > my_patch.txt
Later you can apply this patch, also without a commit, by:
git apply my_patch.txt
But if you are just working locally, a git checkout another_branch -m is good enough to bring all your current uncommit changes to that another_branch, without even patch and apply.