git diff is not very helpful for annexed files.

How about a git annex diff command that allows to compare two versions of an annexed file?

Should be relatively simple, only there would have to be a way to deal with the situation where not both versions are present in the repository. Either abort with a message showing the command you need to run to get the missing version(s). Or even interactively volunteer to get it automatically, asking the user for confirmation.

Of course you wouldn't want to diff two large files, but with git annex assistant, all files are annexed by default (right?), so this would be useful.

There might already be a way to easily diff two versions of an annexed file which I'm missing -- in that case please point me to it! :)

> [[done]]; rather than adding a `git annex diff`, I made git-annex be able to be used as a git diff driver command,
> which in turn can run some third-party external diff driver that does
> some smart handling of binary files.
