time/format: avoid growslice in time.String()/time.GoString()

Pre-allocate the slice of buf with enough capacity
to avoid growslice calls.

benchmark                   old ns/op     new ns/op     delta
BenchmarkTimeString-4       493           409           -17.12%
BenchmarkTimeGoString-4     309           182           -41.30%

benchmark                   old allocs     new allocs     delta
BenchmarkTimeString-4       5              3              -40.00%
BenchmarkTimeGoString-4     4              1              -75.00%

benchmark                   old bytes     new bytes     delta
BenchmarkTimeString-4       152           128           -15.79%
BenchmarkTimeGoString-4     248           80            -67.74%

Change-Id: I64eabe2ab0b3d4a846453c2e8e548a831d720b8c
Reviewed-on: https://go-review.googlesource.com/c/go/+/343971
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Trust: Alexander Rakoczy <[email protected]>

Author: korzhao

src/time/format.go

@@ -479,7 +479,7 @@ func (t Time) String() string {
 		}
 		m1, m2 := m2/1e9, m2%1e9
 		m0, m1 := m1/1e9, m1%1e9
-		var buf []byte
+		buf := make([]byte, 0, 24)
 		buf = append(buf, " m="...)
 		buf = append(buf, sign)
 		wid := 0
@@ -498,7 +498,8 @@ func (t Time) String() string {
 // GoString implements fmt.GoStringer and formats t to be printed in Go source
 // code.
 func (t Time) GoString() string {
-	buf := []byte("time.Date(")
+	buf := make([]byte, 0, 70)
+	buf = append(buf, "time.Date("...)
 	buf = appendInt(buf, t.Year(), 0)
 	month := t.Month()
 	if January <= month && month <= December {