git-crypt diff as textconv for git-blame

Hello! I notice that `git blame` shows encrypted file contents as a single line. [Here's an example session with a fresh repo](https://gist.github.com/jasonm/bd59327e207068edf64f9547949e3ae0). There appears to be support for `textconv` under `git-blame` ([git documentation](https://git.wiki.kernel.org/index.php/Textconv#git_blame) and [git test case](https://github.com/git/git/blob/master/t/t8006-blame-textconv.sh)).

I'm not sure if this is expected or not. I think that the `git blame textconv_demo.odt` example output on that "git documentation" page is what we'd ideally see. Is this something that perhaps could work with a different configuration on my end, or something that could be contributed to `git-crypt`?

Thanks!