ancestor: silence multiple ancestor warning outside of merge (issue4234)

The current situation is a bit of a layering violation as
merge-specific knowledge is pushed down to lower layers and leaks
merge assumptions into other code paths.

Here, we simply silence the warning with a hack. Both the warning and
the hack will probably go away in the near future when bid merge is
made the default.

mercurial/context.py

@@ -394,7 +394,7 b' class changectx(basectx):'
         return filectx(self._repo, path, fileid=fileid,
                        changectx=self, filelog=filelog)
 
-    def ancestor(self, c2):
+    def ancestor(self, c2, warn=False):
         """
         return the "best" ancestor context of self and c2
         """
@@ -415,12 +415,13 b' class changectx(basectx):'
                     break
             else:
                 anc = self._repo.changelog.ancestor(self._node, n2)
-            self._repo.ui.status(
-                (_("note: using %s as ancestor of %s and %s\n") %
-                 (short(anc), short(self._node), short(n2))) +
-                ''.join(_("      alternatively, use --config "
-                          "merge.preferancestor=%s\n") %
-                        short(n) for n in sorted(cahs) if n != anc))
+            if warn:
+                self._repo.ui.status(
+                    (_("note: using %s as ancestor of %s and %s\n") %
+                     (short(anc), short(self._node), short(n2))) +
+                    ''.join(_("      alternatively, use --config "
+                              "merge.preferancestor=%s\n") %
+                            short(n) for n in sorted(cahs) if n != anc))
         return changectx(self._repo, anc)
 
     def descendant(self, other):

mercurial/merge.py

@@ -992,7 +992,7 b' def update(repo, node, branchmerge, forc'
                 cahs = repo.changelog.commonancestorsheads(p1.node(), p2.node())
                 pas = [repo[anc] for anc in (sorted(cahs) or [nullid])]
             else:
-                pas = [p1.ancestor(p2)]
+                pas = [p1.ancestor(p2, warn=True)]
 
         fp1, fp2, xp1, xp2 = p1.node(), p2.node(), str(p1), str(p2)