diff --git a/mercurial/merge.py b/mercurial/merge.py
--- a/mercurial/merge.py
+++ b/mercurial/merge.py
@@ -88,12 +88,9 @@ def update(repo, node, branchmerge=False
         if modified or added or removed:
             raise util.Abort(_("outstanding uncommitted changes"))
 
-    m1n = repo.changelog.read(p1)[0]
-    m2n = repo.changelog.read(p2)[0]
-    man = repo.manifest.ancestor(m1n, m2n)
-    m1 = repo.manifest.read(m1n).copy()
-    m2 = repo.manifest.read(m2n).copy()
-    ma = repo.manifest.read(man)
+    m1 = repo.changectx(p1).manifest().copy()
+    m2 = repo.changectx(p2).manifest().copy()
+    ma = repo.changectx(pa).manifest()
 
     if not force:
         for f in unknown:
@@ -108,7 +105,7 @@ def update(repo, node, branchmerge=False
     repo.ui.debug(_(" overwrite %s branchmerge %s partial %s linear %s\n") %
                   (overwrite, branchmerge, bool(partial), linear_path))
     repo.ui.debug(_(" ancestor %s local %s remote %s\n") %
-                  (short(man), short(m1n), short(m2n)))
+                  (short(p1), short(p2), short(pa)))
 
     action = {}
     forget = []
diff --git a/tests/test-merge7.out b/tests/test-merge7.out
--- a/tests/test-merge7.out
+++ b/tests/test-merge7.out
@@ -23,7 +23,7 @@ added 1 changesets with 1 changes to 1 f
 merge: warning: conflicts during merge
 resolving manifests
  overwrite None branchmerge True partial False linear False
- ancestor 055d847dd401 local 2eded9ab0a5c remote 84cf5750dd20
+ ancestor 451c744aabcc local a070d41e8360 remote faaea63e63a9
  test.txt versions differ, resolve
 merging test.txt
 resolving test.txt
diff --git a/tests/test-up-local-change.out b/tests/test-up-local-change.out
--- a/tests/test-up-local-change.out
+++ b/tests/test-up-local-change.out
@@ -16,7 +16,7 @@ summary:     1
 
 resolving manifests
  overwrite False branchmerge False partial False linear True
- ancestor a0c8bcbbb45c local a0c8bcbbb45c remote 1165e8bd193e
+ ancestor 33aaa84a386b local 802f095af299 remote 33aaa84a386b
  a versions differ, resolve
 remote created b
 merging a
@@ -32,7 +32,7 @@ summary:     2
 
 resolving manifests
  overwrite False branchmerge False partial False linear True
- ancestor a0c8bcbbb45c local 1165e8bd193e remote a0c8bcbbb45c
+ ancestor 802f095af299 local 33aaa84a386b remote 33aaa84a386b
 remote deleted b
 removing b
 0 files updated, 0 files merged, 1 files removed, 0 files unresolved
@@ -50,7 +50,7 @@ summary:     1
 
 resolving manifests
  overwrite False branchmerge False partial False linear True
- ancestor a0c8bcbbb45c local a0c8bcbbb45c remote 1165e8bd193e
+ ancestor 33aaa84a386b local 802f095af299 remote 33aaa84a386b
  a versions differ, resolve
 remote created b
 merging a
@@ -101,7 +101,7 @@ abort: outstanding uncommitted changes
 failed
 resolving manifests
  overwrite False branchmerge True partial False linear False
- ancestor a0c8bcbbb45c local 1165e8bd193e remote 4096f2872392
+ ancestor 802f095af299 local 030602aee63d remote 33aaa84a386b
  a versions differ, resolve
  b versions differ, resolve
 merging a
diff --git a/tests/test-update-reverse.out b/tests/test-update-reverse.out
--- a/tests/test-update-reverse.out
+++ b/tests/test-update-reverse.out
@@ -41,7 +41,7 @@ side1
 side2
 resolving manifests
  overwrite True branchmerge False partial False linear False
- ancestor 8515d4bfda76 local 1c0f48f8ece6 remote 0594b9004bae
+ ancestor ded32b0db104 local 221226fb2bd8 remote 537353581d3d
 remote deleted side2, clobbering
 remote deleted side1, clobbering
 remote created main