internal/strs: avoid unnecessary allocations in Builder

The grow() method in func did create a new internal byte slice with a
specific size instead of a specific capacity. As the call following
grow() is append() we ended up with a slice larger than needed.

It affects the memory consumption of programs that import generated
protobuf files. As an example with the following:

-- go.mod --
module x

go 1.20

require cloud.google.com/go/appengine v1.6.0
-- main.go --
package main

import _ "cloud.google.com/go/appengine/apiv1/appenginepb"

func main() {}

Running the following a few times:
$ go mod tidy && GODEBUG=inittrace=1 go run . 2>&1 | grep appenginepb

Before:
init cloud.google.com/go/appengine/apiv1/appenginepb @3.4 ms, 0.23 ms clock, 204624 bytes, 231 allocs
init cloud.google.com/go/appengine/apiv1/appenginepb @3.2 ms, 0.17 ms clock, 204688 bytes, 231 allocs
init cloud.google.com/go/appengine/apiv1/appenginepb @2.5 ms, 0.15 ms clock, 204400 bytes, 230 allocs
init cloud.google.com/go/appengine/apiv1/appenginepb @2.5 ms, 0.16 ms clock, 205552 bytes, 234 allocs

After:
init cloud.google.com/go/appengine/apiv1/appenginepb @3.3 ms, 0.19 ms clock, 143440 bytes, 226 allocs
init cloud.google.com/go/appengine/apiv1/appenginepb @2.7 ms, 0.16 ms clock, 144368 bytes, 229 allocs
init cloud.google.com/go/appengine/apiv1/appenginepb @2.8 ms, 0.16 ms clock, 144304 bytes, 229 allocs
init cloud.google.com/go/appengine/apiv1/appenginepb @3.1 ms, 0.16 ms clock, 142864 bytes, 224 allocs

Change-Id: If4ece5d70d6bd9de8a758cb29ce9dffc741c4951
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/465115
Reviewed-by: Joseph Tsai <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
Reviewed-by: Michael Stapelberg <[email protected]>

internal/strs/strings_unsafe.go

@@ -87,7 +87,7 @@ func (sb *Builder) grow(n int) {
 	// Unlike strings.Builder, we do not need to copy over the contents
 	// of the old buffer since our builder provides no API for
 	// retrieving previously created strings.
-	sb.buf = make([]byte, 2*(cap(sb.buf)+n))
+	sb.buf = make([]byte, 0, 2*(cap(sb.buf)+n))
 }
 
 func (sb *Builder) last(n int) string {