Add command to decrypt secrets file to stdout for git diff

[elikoga]

Solves #2, which is possible since we changed the file endings of the batou secrets.

Running

git config diff.batou.textconv "./batou secrets decrypttostdout"
echo "*.gpg diff=batou" >> .gitattributes
echo "*.age diff=batou" >> .gitattributes
echo "*.age-diffable diff=batou" >> .gitattributes

within a batou deployment project gives the desired result.