add diff.openssl-x509-text.textconv for files containing x509 certs

gitconfig

@@ -36,6 +36,9 @@
 [diff "hexdump"]
 	textconv = od -t x1z --
 
+[diff "openssl-x509-text"]
+	textconv = "sh -c 'while openssl x509 -noout -text; do echo; done' <\"$1\" 2>&1"
+
 [diff "pgpdump"]
 	textconv = pgpdump