path/filepath: EvalSymlinks, incorrect traversal of relative paths [1.12 backport]

[gopherbot]

@ianlancetaylor requested issue #30520 to be considered for backport to the next 1.12 minor release.

@gopherbot please open backport to 1.12.

[gopherbot]

Change https://golang.org/cl/165197 mentions this issue: [release-branch.go1.12] path/filepath: don't discard .. in EvalSymlinks

[gopherbot]

Closed by merging 0ea7460 to release-branch.go1.12.