context.walk: call with util.all() a generator, not a list

The file set can be large, so avoid going through the entire file set
when a file happens not to be in the context.

mercurial/context.py

@@ -594,7 +594,7 b' class changectx(basectx):'
 
         # avoid the entire walk if we're only looking for specific files
         if fset and not match.anypats():
-            if util.all([fn in self for fn in fset]):
+            if util.all(fn in self for fn in fset):
                 for fn in sorted(fset):
                     if match(fn):
                         yield fn