ui-atom: properly escape delimiter in page link

If the delimiter here is '&' then it needs to be escaped for inclusion
in an attribute.  Use html_attrf() to ensure that this happens (we know
that hex won't need escaping, but this makes it clearer what's
happening.

Signed-off-by: John Keeping <john@keeping.me.uk>

ui-atom.c

@@ -63,7 +63,7 @@ static void add_entry(struct commit *commit, const char *host)
 		html_attr(pageurl);
 		if (ctx.cfg.virtual_root)
 			delim = '?';
-		htmlf("%cid=%s", delim, hex);
+		html_attrf("%cid=%s", delim, hex);
 		html("'/>\n");
 		free(pageurl);
 	}