Notably, don't allocate.

Follow-up to https://golang.org/cl/32451 which added long path
cleaning.

Updates #3358

Change-Id: I89c59cbd660d0a030f31b6acd070fa9f3250683b
Reviewed-on: https://go-review.googlesource.com/32886
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>