secure/precis: small optimization
secure/precis: small optimization

Don't create set for each call to Transform.

Change-Id: I257f67e33aae21899dcc23a974a06e57d2258d91
Reviewed-on: https://go-review.googlesource.com/23594
Run-TryBot: Marcel van Lohuizen <a22d082192f3ccf984644a4f482781f4c2d65180@golang.org>
Reviewed-by: Sam Whited <f16bed56189e249fe4ca8ed10a1ecae60e8ceac0@samwhited.com>
Reviewed-by: Nigel Tao <58ae08b3356928ff9c67562b4719487d54fe8348@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/text