time: simplify stringification of Month

Simplifies https://golang.org/cl/33145
which fixed #17720.

Change-Id: Ib922d493cdc5920832dc95b55094796baca7243e
Reviewed-on: https://go-review.googlesource.com/33194Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/time/time.go

@@ -118,13 +118,9 @@ func (m Month) String() string {
 	if January <= m && m <= December {
 		return months[m-1]
 	}
-	const prefix = "%!Month("
-	buf := make([]byte, 20+len(prefix)+1)
-	buf[len(buf)-1] = ')'
-	n := fmtInt(buf[:len(buf)-1], uint64(m))
-	n -= len(prefix)
-	copy(buf[n:], prefix)
-	return string(buf[n:])
+	buf := make([]byte, 20)
+	n := fmtInt(buf, uint64(m))
+	return "%!Month(" + string(buf[n:]) + ")"
 }
 
 // A Weekday specifies a day of the week (Sunday = 0, ...).