manifest: fix a bug where working copy file 'add' mark was buggy

Because the same dictionary was used to (1) get node from parent and (2) store
annotated version, we could end up with buggy values. For example with a chain
of renames:

  $ hg mv b c
  $ hg mv a b

The value from 'b' would be updated as "<old-a>a", then the value of c would be
updated as "<old-b>a'. With the current dictionary sharing this ends up with:

    '<new-c>' == '<old-a>aa'

This value is double-wrong as we should use '<old-b>' and a single 'a'.

We now use a read-only value for lookup. The 'test-rename.t' test is impacted
because such a chained added file is suddenly detected as such.

mercurial/context.py

@@ -1063,15 +1063,16 b' class committablectx(basectx):'
     def _manifest(self):
         """generate a manifest corresponding to the values in self._status"""
 
-        man = self._parents[0].manifest().copy()
+        man1 = self._parents[0].manifest()
+        man = man1.copy()
         if len(self._parents) > 1:
             man2 = self.p2().manifest()
             def getman(f):
-                if f in man:
-                    return man
+                if f in man1:
+                    return man1
                 return man2
         else:
-            getman = lambda f: man
+            getman = lambda f: man1
 
         copied = self._repo.dirstate.copies()
         ff = self._flagfunc

tests/test-rename.t

@@ -584,7 +584,8 b' overwriting with renames (issue1959)'
   copy from d1/a
   copy to d1/c
   $ hg update -C
-  2 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ rm d1/c # The file was marked as added, so 'hg update' action  was 'forget'
 
 check illegal path components