Fix schema export with @reverse for 1.2.x

Moved from GitHub dgraph/4886

Posted by miko:

The actual bug is described in #4856 and resolved in #4857 for master branch.
This should be backported to 1.2.x (and a new 1.2.x version should be released) because - as I understand - upgrading dgraph to v2.0.0. will require export from v1.2.x, and that will create unusable schemas for v.1.2.1 and earlier versions.