Add "Index: " to the list of things that start a patch

This way we don't get it in the commit message, even if the patch had
been generated by cogito (or CVS, ugh) and people didn't add the proper
"---" marker.

mailinfo.c

@@ -193,7 +193,8 @@ static void handle_rest(void)
 
 	do {
 		if (!memcmp("diff -", line, 6) ||
-		    !memcmp("---", line, 3))
+		    !memcmp("---", line, 3) ||
+		    !memcmp("Index: ", line, 7))
 			out = patchfile;
 
 		fputs(line, out);