Using git, how do I ignore a file in one branch but have it committed in another branch?

This solution appears to work only for certain, patched versions of git. See a new answer pointing to workarounds and another answer and subsequent comments for a hint which versions may work.

I wrote a blog post on how to effectively use the excludesfile for different branches, like one for public github and one for heroku deployment.

Here's the quick and dirty:

$ git branch public_viewing
$ cd .git/
$ touch info/exclude_from_public_viewing
$ echo "path/to/secret/file" > info/exclude_from_public_viewing 

then in the .git/config file add these lines:

[core]
excludesfile = +info/exclude


[branch "public_viewing"]
excludesfile = +info/exclude_from_public_viewing

Now all the global ignore stuff is in the info/exclude file and the branch specific is in the info/exclude_from_public_viewing

Hope that helps!

http://cogniton-mind.tumblr.com/post/1423976659/howto-gitignore-for-different-branches

Important hint: The accepted answer by Cognition.Mind doesn't work (anymore, for several years now, or perhaps for vanilla versions of git); see the comments. A valid answer and workaround can be found here:

https://stackoverflow.com/a/29583813/2157640

Another alternative workaround (working for my particular problem, but demanding manual stash operations or implementation of a hook) would be git stash -u -a. This is tedious when the differences are large.

And finally, the solution I'm now going with is having forked my VM that we hold our development environment in, and set up info/excludes appropriately for the branch, respectively deleting the offending, uncommitted files/folders.

I would strongly advise considering putting those MP3 files on S3. Having them be part of your Heroku push (and thus part of your Heroku slug) will greatly slow down your dyno startup time. Since Heroku uses EC2, if the files are on S3 and are only accessed by your app (if users aren't directly linked to S3) you won't even pay any bandwidth charges, only the charge to store 50MB.