what_changed: the build graph is sorted twice even though the first iteration does not require the targets to be sorted. Removing that sort reduces the call time by about a third in a non scientific test.

Testing Done:
ci.sh passes locally

Reviewed at https://rbcommons.com/s/twitter/r/1291/
Sync of https://github.com/pantsbuild/pants/commit/efbf2a8b91e49ead4043df6c6fc2ba10e9c0edd0