git diff: use `git --verify` to textconv signature-* files

.gitattributes

@@ -0,0 +1,2 @@
+# https://git.hammerle.me/fphammerle/config-git/commit/e20b3bbcfbae3ab357b4da9fefc40c8cbaefc1f2
+signature-* diff=gpg-verify