merge: pass constant cset ancestor to fctx.ancestor

mercurial/merge.py

@@ -242,8 +242,13 b' def manifestmerge(repo, p1, p2, pa, over'
 def actionkey(a):
     return a[1] == 'r' and -1 or 0, a
 
-def applyupdates(repo, action, wctx, mctx):
-    "apply the merge action list to the working directory"
+def applyupdates(repo, action, wctx, mctx, actx):
+    """apply the merge action list to the working directory
+
+    wctx is the working copy context
+    mctx is the context to be merged into the working copy
+    actx is the context of the common ancestor
+    """
 
     updated, merged, removed, unresolved = 0, 0, 0, 0
     ms = mergestate(repo)
@@ -263,7 +268,7 b' def applyupdates(repo, action, wctx, mct'
             repo.ui.debug("preserving %s for resolve of %s\n" % (f, fd))
             fcl = wctx[f]
             fco = mctx[f2]
-            fca = fcl.ancestor(fco) or repo.filectx(f, fileid=nullrev)
+            fca = fcl.ancestor(fco, actx) or repo.filectx(f, fileid=nullrev)
             ms.add(fcl, fco, fca, fd, flags)
             if f != fd and move:
                 moves.append(f)
@@ -505,7 +510,7 b' def update(repo, node, branchmerge, forc'
         if not partial:
             repo.hook('preupdate', throw=True, parent1=xp1, parent2=xp2)
 
-        stats = applyupdates(repo, action, wc, p2)
+        stats = applyupdates(repo, action, wc, p2, pa)
 
         if not partial:
             repo.dirstate.setparents(fp1, fp2)