Can I get a patch-compatible output from git-diff?

I am doing something very simple wrong. I'm trying to prepare an ordinary patch file, so I can reapply some changes:

$ git diff > before
$ git diff something_here > save.patch
$ git checkout . 
$ patch < save.patch
$ git diff > after
$ diff before after
$

With something_here blank it almost works, but the file names aren't right. I think I'm just I'm missing some option.

In real life, I am going to do a merge after the checkout, so the patch might fail there, but you see what I'm getting at.

Edit My fault here for asking the wrong question. The actual question is, I want to save my changes away, do a merge, then re-apply the changes, if possible? I asked it the wrong way because I am used to using patch to solve these sorts of problems and git diff looked like that's what it wanted me to do.

Charles Bailey's comment had the right answer. For me, git-apply is the right thing to do (git-stash looks more heavy-weight than I need and rebasing and bundles is definitely beyond my current skill level.) I'm going to accept the answer Charles gave (because you can't accept a comment). Thanks for all the suggestions.

Edit, 6 years later As anyone familiar with the subject knows, I over-estimated the difficulty of git stash. Pretty much every day or so, I will use the following sequence:

$ git stash
$ git merge
$ git stash pop

Just use -p1: you will need to use -p0 in the --no-prefix case anyway, so you can just leave out the --no-prefix and use -p1:

$ git diff > save.patch
$ patch -p1 < save.patch

$ git diff --no-prefix > save.patch
$ patch -p0 < save.patch

If you want to use patch you need to remove the a/ b/ prefixes that git uses by default. You can do this with the --no-prefix option (you can also do this with patch's -p option):

git diff --no-prefix [<other git-diff arguments>]

Usually though, it is easier to use straight git diff and then use the output to feed to git apply.

Most of the time I try to avoid using textual patches. Usually one or more of temporary commits combined with rebase, git stash and bundles are easier to manage.

For your use case I think that stash is most appropriate.

# save uncommitted changes
git stash

# do a merge or some other operation
git merge some-branch

# re-apply changes, removing stash if successful
# (you may be asked to resolve conflicts).
git stash pop