math/rand: revert Perm changes?

[rsc]

I encouraged Josh to change Perm to match Shuffle, but it's a potentially breaking change and while writing the release notes I am reluctant to describe it. We should probably revert the behavioral change in Perm.

[dsnet]

I support reverting rand.Perm changes. It is the cause of a few failures inside Google, and the fix for them is not easy.

[mvdan]

The CL in question is https://golang.org/cl/55972, if anyone was wondering (like myself).

[gopherbot]

Change https://golang.org/cl/78195 mentions this issue: Revert "math/rand: make Perm match Shuffle"