treemanifest: speed up commit using dirty flag

We currently avoid saving a treemanifest revision if it's the same as
one of it's parents. This is checked by comparing the generated text
for all three versions. Let's avoid that when possible by comparing
the nodeids for clean (not dirty) nodes.

On the Mozilla repo, this speeds up commit from 2.836s to 2.343s.

mercurial/manifest.py

@@ -759,6 +759,9 b' class treemanifest(object):'
         _diff(self, m2)
         return result
 
+    def unmodifiedsince(self, m2):
+        return not self._dirty and not m2._dirty and self._node == m2._node
+
     def parse(self, text, readsubtree):
         for f, n, fl in _parse(text):
             if fl == 'd':
@@ -951,13 +954,16 b' class manifest(revlog.revlog):'
         return n
 
     def _addtree(self, m, transaction, link, m1, m2):
+        # If the manifest is unchanged compared to one parent,
+        # don't write a new revision
+        if m.unmodifiedsince(m1) or m.unmodifiedsince(m2):
+            return m.node()
         def writesubtree(subm, subp1, subp2):
             sublog = self.dirlog(subm.dir())
             sublog.add(subm, transaction, link, subp1, subp2, None, None)
         m.writesubtrees(m1, m2, writesubtree)
         text = m.dirtext(self._usemanifestv2)
-        # If the manifest is unchanged compared to one parent,
-        # don't write a new revision
+        # Double-check whether contents are unchanged to one parent
         if text == m1.dirtext(self._usemanifestv2):
             n = m1.node()
         elif text == m2.dirtext(self._usemanifestv2):