Patching

$ git branch my_fix
$ git checkout my_fix
[edit]
$ git commit -a
$ git format-patch HEAD~1 --stdout >my_fix.patch

Or if you don't want to save this commit at all, revert after creating patch:

git reset --hard HEAD~1

(Committing is good for storing the commit message.) Transfer my_fix.patch to someone who does:

$ git am my_fix.patch