easier 'diff'

It would be nice to be able to more easily 'diff' secrets between git commits. `textconv` gets us part of the way there: if you add a `.gitattributes` with `*.age diff=age` and then edit `.git/config` to: ``` [diff "age"] textconv = age -d -i ~/.ssh/id_ed25519 ``` ... this sorta seems to work, except in my case ~/.ssh/id_ed25519 is password-protected. This causes 2 problems: * it seems the pager somehow interferes with my password entry, so it's rejected often * the pager is invoked for *every* age file, not just the ones that have changed - so it'll prompt me for my password way too often Upstream age is not a fan of encrypted keys in the first place (https://github.com/FiloSottile/age/issues/366#issuecomment-981610446) If this worked more smoothly it might be worth documenting it and perhaps providing an 'agenix wrapper', but for now I'll just leave my learnings here ;)