Question: Is it possible to selectively apply changes from one branch to another with Git?

More specifically, I use a public `dev` branch for GitHub and a private `master` branch for deployment. When changes are made to one branch they will need to be applied to the other, but some lines of code need to stay different. In my case it's a few css classes and a feed.

I'm new to Git but I've made my research :

* `git merge --no-commit --no-ff` can be used followed by a `git mergetool` to chose what I want in case of a conflict. The problem is that it only works for conflicts Git can't automatically merge, so what I want to stay different gets replaced before I get the chance to use my mergetool.
* `git difftool --cached` is useful as it allows me to see the differences, but I need to copy what I want to keep from there and manually replace it with a text editor, as I cannot simply choose and save like I can with mergetool.
* `git cherry-pick` seems to apply a specified commit to another, but what I want to stay different may be scattered to different commits, and these commits may not only include what I want to stay different. I cannot see this working unless I make millions of commits that would drive me mad.

Also to be clear, **I don't want one branch to become another**, what seems to be the case with a merge. **I want two separate branches with their respective differences and apply changes from one to the other.**

Is there a better workflow that would allow me to keep a development and a deployment version by applying their changes and keeping a few differences? I don't mind using separate repositories or different tools if it leads to a solution.

Answer: 
I've also found out about patching : 

To create the patch : `git diff dev > master.patch`

To apply it : `patch < master.patch`
I don't think there is a way to pick parts of a commit from the same file. I would say you need to simply re-factor your code to move such parts in different files.

BTW, if you want to take some files from a commit, you can use cherry-pick with a combination of other commands as explained [here](https://stackoverflow.com/a/1526093/2279816).