memctx: calculate manifest more efficiently

Before this patch, "memctx._manifest" updates all entries in the
(parent) manifest. But this is inefficiency, because almost all files
may be clean in that context.

On the other hand, just updating entries for changed "files" specified
at construction causes unexpected abortion, when there is at least one
newly removed file (see issue4470 for detail).

To calculate manifest more efficiently, this patch replaces
"pman.iteritems()" for the loop by "self._status.modified" to avoid
updating entries for clean or removed files

Examination of removal is also omitted, because removed files aren't
treated in this loop (= "self[f]" returns not None always).

mercurial/context.py

@@ -1633,10 +1633,9 b' class memctx(committablectx):'
 
         # keep this simple for now; just worry about p1
         pctx = self._parents[0]
-        pman = pctx.manifest()
         man = pctx.manifest().copy()
 
-        for f, fnode in pman.iteritems():
+        for f in self._status.modified:
             p1node = nullid
             p2node = nullid
             p = pctx[f].parents() # if file isn't in pctx, check p2?
@@ -1644,12 +1643,7 b' class memctx(committablectx):'
                 p1node = p[0].node()
                 if len(p) > 1:
                     p2node = p[1].node()
-            fctx = self[f]
-            if fctx is None:
-                # removed file
-                del man[f]
-            else:
-                man[f] = revlog.hash(fctx.data(), p1node, p2node)
+            man[f] = revlog.hash(self[f].data(), p1node, p2node)
 
         for f in self._status.added:
             man[f] = revlog.hash(self[f].data(), nullid, nullid)