net: combine append calls in reverseaddr

Combined appends lead to fewer machine code and faster performance.
Some may even say that it makes code more readable.

Running revAddrTests over reverseaddr gives measurable improvements:

	name              old time/op  new time/op  delta
	ReverseAddress-8  4.10µs ± 3%  3.94µs ± 1%  -3.81%  (p=0.000 n=10+9)

Change-Id: I9bda7a20f802bcdffc6e948789765d04c6da04e7
Reviewed-on: https://go-review.googlesource.com/117615
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/dnsclient.go

@@ -27,10 +27,10 @@ func reverseaddr(addr string) (arpa string, err error) {
 	// Add it, in reverse, to the buffer
 	for i := len(ip) - 1; i >= 0; i-- {
 		v := ip[i]
-		buf = append(buf, hexDigit[v&0xF])
-		buf = append(buf, '.')
-		buf = append(buf, hexDigit[v>>4])
-		buf = append(buf, '.')
+		buf = append(buf, hexDigit[v&0xF],
+			'.',
+			hexDigit[v>>4],
+			'.')
 	}
 	// Append "ip6.arpa." and return (buf already has the final .)
 	buf = append(buf, "ip6.arpa."...)