Local changes would be overwritten by checkout

[craigmdennis]

I'm not sure if this is best posted here or on middleman-imageoptim.

When I run imageoptim it generates an imageoptim.manifest.yml to prevent optimising images it's already done. However the following error occurs when running deploy:

error: Your local changes to the following files would be overwritten by checkout:
    imageoptim.manifest.yml
Please, commit your changes or stash them before you can switch branches.
Aborting

The only way to resolve the error is to delete the /build/ folder entirely