context: stop setting None for modified or added nodes

Instead use a magic value, so that we can identify modified or added
nodes correctly when using manifest.diff().

Thanks to Martin von Zweigbergk for catching that we have to update
_buildstatus as well. That part eluded my debugging for some time.

mercurial/context.py

@@ -17,6 +17,11 b' import revlog'
 
 propertycache = util.propertycache
 
+# Phony node value to stand-in for new files in some uses of
+# manifests. Manifests support 21-byte hashes for nodes which are
+# dirty in the working copy.
+_newnode = '!' * 21
+
 class basectx(object):
     """A basectx object represents the common logic for its children:
     changectx: read-only context that is already present in the repo,
@@ -128,7 +133,7 b' class basectx(object):'
                 if (fn not in deletedset and
                     ((fn in withflags and mf1.flags(fn) != mf2.flags(fn)) or
                      (mf1[fn] != mf2node and
-                      (mf2node or self[fn].cmp(other[fn]))))):
+                      (mf2node != _newnode or self[fn].cmp(other[fn]))))):
                     modified.append(fn)
                 elif listclean:
                     clean.append(fn)
@@ -1408,7 +1413,7 b' class workingctx(committablectx):'
         mf = self._repo['.']._manifestmatches(match, s)
         modified, added, removed = s[0:3]
         for f in modified + added:
-            mf[f] = None
+            mf[f] = _newnode
             mf.setflag(f, self.flags(f))
         for f in removed:
             if f in mf: