Summary:
Make the error cleaner and more actionable. We don't autopull the commit
because the revset layer might be not ready for it (ex. it expects commit
graph to be immutable and might have done some calculations based on the
old graph already).
Reviewed By: sfilipco
Differential Revision: D20845159
fbshipit-source-id: c51f2f52c612ff14a88fb891c10d1faad1094635
