Summary:
`repair.strip` removes required revisions and their children unconditionally.
That means innocent children may be removed. This patch fixes it.
We may want to have an in-core function for this feature to not strip innocent
children.
Thanks Zibi Braniecki from Mozilla for reporting [1]!
[1]: https://bitbucket.org/facebook/hg-experimental/issues/6/hg-absorb-merges-diverged-commits
Test Plan: Added a test
Reviewers: #mercurial, durham
Reviewed By: durham
Subscribers: mjpieters
Differential Revision: https://phabricator.intern.facebook.com/D5073342
Signature: t1:5073342:1494972231:ca2a9881415119047771c8f35e059f4bfb1749e5
