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)