Auto merge of #4714 - Mark-Simulacrum:frozen-freezes, r=alexcrichton

Do not update semantically equivalent lockfiles with --frozen/--locked.

A previous patch in #4684 attempted to fix this, but didn't work for the
case where the [root] crate wasn't the first crate in the sorted package
array.

cc @matklad -- fixes a problem noted in https://github.com/rust-lang/cargo/pull/4684#discussion_r147824900 which appears to have gone unfixed

r? @alexcrichton

Beta backport will be posted as soon as this is reviewed. Merges cleanly.