narrow: restrict manifest iteration by using manifest.walk(matcher)

This is only for root nodes, so it shouldn't really matter (they're
rarely huge), but seems cleaner.

Differential Revision: https://phab.mercurial-scm.org/D2234

hgext/narrow/narrowbundle2.py

@@ -152,7 +152,7 b' def _computeellipsis(repo, common, heads'
             else:
                 # For a root node, we need to include the node if any
                 # files in the node match the narrowspec.
-                needed = any(match(f) for f in curmf)
+                needed = any(curmf.walk(match))
 
         if needed:
             for head in ellipsisheads[rev]: