corruption: backout changeset
49fd21f32695 (issue6528)
inverting the parent is masking copy information leading to bad content being
fetched and bad status result.
Since 49fd21f32695, exchange can actively swap these parent corrupting existing
changesets and triggering the corruption.
Data corruption are considered critical so backing this out and doing and
unscheduled release seems in order.
Differential Revision:
https://phab.mercurial-scm.org/D10995
