batcheval: check GCThreshold in RevertRange command

I think somehow I got mixed up about when a key is GC'ed in that original comment.
So that we can time-travel to any time within GC window, we already only GC a key
when there is a newer _expired_ key over it, and the requirements for being able
to revert to a time are no different than being able to read at that time, so the
same GCThreshold check we do elsewhere should also be good enough here.

Release note: none.