absorb: do not strip innocent children

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

hgext3rd/absorb/__init__.py

@@ -816,6 +816,11 @@ class fixupstate(object):
 
     def _stripoldcommits(self):
         nodelist = self.replacemap.keys()
+        # make sure we don't strip innocent children
+        revs = self.repo.revs('%ln - (::(heads(%ln::)-%ln))', nodelist,
+                              nodelist, nodelist)
+        tonode = self.repo.changelog.node
+        nodelist = [tonode(r) for r in revs]
         if nodelist:
             repair.strip(self.repo.ui, self.repo, nodelist)
 

tests/test-absorb-strip.t

@@ -0,0 +1,45 @@
+Do not strip innocent children. See https://bitbucket.org/facebook/hg-experimental/issues/6/hg-absorb-merges-diverged-commits
+
+  $ cat >> $HGRCPATH << EOF
+  > [extensions]
+  > absorb=$TESTDIR/../hgext3rd/absorb
+  > drawdag=$RUNTESTDIR/drawdag.py
+  > EOF
+
+  $ hg init
+  $ hg debugdrawdag << EOF
+  > E
+  > |
+  > D F
+  > |/
+  > C
+  > |
+  > B
+  > |
+  > A
+  > EOF
+
+  $ hg up E -q
+  $ echo 1 >> B
+  $ echo 2 >> D
+  $ hg absorb
+  saved backup bundle to * (glob)
+  2 of 2 chunk(s) applied
+
+  $ hg log -G -T '{desc}'
+  @  E
+  |
+  o  D
+  |
+  o  C
+  |
+  o  B
+  |
+  | o  F
+  | |
+  | o  C
+  | |
+  | o  B
+  |/
+  o  A
+