merge - Git: merging public and private branches while while keeping certain files intact in both branches

Question

I've read a few git questions here, but could not find an answer to this one:

I have a public and a private branches where I want to allow certain files to diverge.

Those are configuration files with passwords and my local customizations.

I do want to be able to merge the branches both ways: from private to public and back, but I do not want to ever have those specific files merged automatically.

Is there a way to set up git this way? I'd love to find an automated solution :) - so that merging could be done as usual.

---

EDIT: here's the solution that worked for me (Thanks to VonC for the advice on gitattribute)

the only unexpected thing for me was that "merge protection" starts working only after files have diverged in the two branches, not immediately after the following configuration was applied

.gitattributes (track with git if you want to share this) or .git/info/attributes:

file1      merge=keepmine
path/file2     merge=keepmine

keepmine is the named custom merge manager that is just a do-nothing command called instead of the internal merge driver on selected files, which is set up below

When merging from private to public branch I usually do git merge --squash private. That way private edits won't get into the git history on the public branch.

.git/config:

#public repository
[remote "origin"]
    fetch = +refs/heads/*:refs/remotes/origin/*
    url = <public repo git url> 

#private repository
#has to set up with git init and populated with the initial commit to branch mybranch
[remote "private"]
    push = +:
    url = /path/to/local/private/repo 
[merge "keepmine"]
    name = dont_merge_selected_files
    driver = echo %O %A %B 
[branch "master"]
    remote = origin
    merge = refs/heads/master 

#private branch settings
[branch "mybranch"]
    remote = private
    merge = refs/heads/mybranch

if there's a way to improve this please comment

Answer 1

To be on the safe side, you can add a git attribute (see here for an example) for those private files.

That way, you can define a script (a "merge manager") which will ensure the file including private informations will remain empty (or with a public content) if merged on the public branch, while keeping its local content if merged to the private branch.
It means you can merge/rebase without thinking about that file.