The required slice capacity is already known. Thus, preallocate a slice with the correct capacity before appending to it.

Change-Id: I45ac2c5f1701caeb3dda20451d371713ae7e7365
GitHub-Last-Rev: 2bf575be65e9a449322540270988eaf87cec4245
GitHub-Pull-Request: golang/go#34602
Reviewed-on: https://go-review.googlesource.com/c/go/+/197917


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>