grep: speed up `hg grep --all-files some/path` by using ctx.matches(match)

ctx.matches(match) avoids walking unintersting parts of the tree when
using tree manifests. That can make a very big difference when
grepping in a small subset of the tree (2.0s -> 0.7s in my case, but
can of course be made more extreme by picking a smaller subset of
files).

Differential Revision: https://phab.mercurial-scm.org/D7820

mercurial/cmdutil.py

@@ -2429,10 +2429,14 b' def walkchangerevs(repo, match, opts, pr'
 
                     def fns_generator():
                         if allfiles:
-                            fiter = iter(ctx)
+
+                            def bad(f, msg):
+                                pass
+
+                            for f in ctx.matches(matchmod.badmatch(match, bad)):
+                                yield f
                         else:
-                            fiter = ctx.files()
-                        for f in fiter:
+                            for f in ctx.files():
                             if match(f):
                                 yield f