Comparing buffers with vimdiff

Vim's diff mode allows us to easily compare the contents of two (or more) buffers. We can start Vim in diff mode using the `vimdiff` command, or if Vim is already running we can switch to diff mode using the `:diffthis` command. The beauty of the `:diffthis` command is that it works with unnamed buffers, whereas `vimdiff` can only work with files.