changectx: increase perf of walk function

When running 'hg cat -r . <file>' it was doing an expensive ctx.walk(m) which
applied the regex to every file in the manifest.

This changes changectx.walk to iterate over just the files in the regex, if no
other patterns are specified. This cuts hg cat time by 50% in our repo and
probably benefits a few other commands as well.

mercurial/context.py

@@ -410,6 +410,15 b' class changectx(basectx):'
         # for dirstate.walk, files=['.'] means "walk the whole tree".
         # follow that here, too
         fset.discard('.')
+
+        # 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]):
+                for fn in sorted(fset):
+                    if match(fn):
+                        yield fn
+                raise StopIteration
+
         for fn in self:
             if fn in fset:
                 # specified pattern is the exact name