We're always going to add stdin, stdout, and stderr to childFiles, so
its length will be at least three. The final length will be those three
elements plus however many files were given via ExtraFiles.

Allocate for that final length directly, saving two slice growth allocs
in the common case where ExtraFiles is empty.

name        old time/op    new time/op    delta
ExecEcho-8     435µs ± 0%     435µs ± 0%    ~     (p=0.394 n=6+6)

name        old alloc/op   new alloc/op   delta
ExecEcho-8    6.39kB ± 0%    6.37kB ± 0%  -0.39%  (p=0.002 n=6+6)

name        old allocs/op  new allocs/op  delta
ExecEcho-8      36.0 ± 0%      34.0 ± 0%  -5.56%  (p=0.002 n=6+6)

Change-Id: Ib702c0da1e43f0a55ed937af6d45fca6a170e8f3
Reviewed-on: https://go-review.googlesource.com/c/164898
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>