manifest.walk: special-case match.always() for speed

This cuts down the run time of

  hg files -r . > /dev/null

from ~0.850s to ~0.780s on the Firefox repo. Note that
manifest.matches() already has the corresponding optimization.

mercurial/manifest.py

@@ -222,6 +222,11 b' class manifestdict(object):'
 
         It also reports nonexistent files by marking them bad with match.bad().
         '''
+        if match.always():
+            for f in iter(self):
+                yield f
+            return
+
         fset = set(match.files())
 
         # avoid the entire walk if we're only looking for specific files
@@ -607,6 +612,11 b' class treemanifest(object):'
 
         It also reports nonexistent files by marking them bad with match.bad().
         '''
+        if match.always():
+            for f in iter(self):
+                yield f
+            return
+
         fset = set(match.files())
 
         for fn in self._walk(match):