context: generate file ancestors in reverse revision order (issue2642)

The related() function in copies assumes that revisions are generated
in reverse revision order, so it was getting confused in some situations.

mercurial/context.py

@@ -550,10 +550,14 b' class filectx(object):'
         return None
 
     def ancestors(self):
-        seen = set(str(self))
+        seen = set()
         visit = [self]
         while visit:
-            for parent in visit.pop(0).parents():
+            parents = visit.pop(0).parents()
+            if len(parents) > 1 and parents[1].rev() > parents[0].rev():
+                # make sure we return ancestors in reverse revision order
+                parents = reversed(parents)
+            for parent in parents:
                 s = str(parent)
                 if s not in seen:
                     visit.append(parent)

tests/test-rename-merge1.t

@@ -99,3 +99,29 b" We'd rather not warn on divergent rename"
    b3
    b4
   2 files updated, 0 files merged, 1 files removed, 0 files unresolved
+
+Check for issue2642
+
+  $ hg init t
+  $ cd t
+
+  $ echo c0 > f1
+  $ hg ci -Aqm0
+
+  $ hg up null -q
+  $ echo c1 > f1 # backport
+  $ hg ci -Aqm1
+  $ hg mv f1 f2
+  $ hg ci -qm2
+
+  $ hg up 0 -q
+  $ hg merge 1 -q --tool internal:local
+  $ hg ci -qm3
+
+  $ hg merge 2
+  merging f1 and f2 to f2
+  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+
+  $ cat f2
+  c0