Add more environment variables

This change adds REPO_LREV and REPO_RREV, both of which are set in `repo
forall`[1]. This also updates the doc for REPO_REMOTE to reflect the
reality since it does not always return the URL.

1: https://source.android.com/source/using-repo.html#forall

Bug: 33219807
Test: Added a pre-upload that called `env`, saw new fields.
Change-Id: Ifec0a3e05bc6832ead61baa5ccb0b5d2c29d948e