Symlinked directories should be protected against accidental writes

[jasoncodes]

My freshrc contains the following:

fresh vim/colors --file=~/.vim/colors/

This builds ~/.fresh/build/vim-colors and symlinks it to ~/.vim/colors. As a result, any new files created in ~/.vim/colors are silently lost when fresh is next ran. We should make these directories read only to prevent accidental loss.

Care should be taken to test the cleanup code for ~/.fresh/build.old. Automated tests for this part is probably not required unless existing code fails.