walkchangerevs: obey allfiles parameter when taking the slow path

When walkchangerevs sees that there's a pattern, it hits the slow
path. The slow path in turn reverts to the old dumb grep behaviour of
only looking at files changed at each revision. Therefore, a command
such as

    hg grep -l --all-files '.*' 'glob:**'

would show you all the nonempty files touched by the current revision.

This modifies that behaviour to look at the manifest at each revision
instead of the changed files in case that --all-files was requested.

mercurial/cmdutil.py

@@ -1961,6 +1961,9 b' def walkchangerevs(repo, match, opts, pr'
                 else:
                     self.revs.discard(value)
                     ctx = change(value)
+                    if allfiles:
+                        matches = list(ctx.manifest().walk(match))
+                    else:
                     matches = [f for f in ctx.files() if match(f)]
                     if matches:
                         fncache[value] = matches

tests/test-grep.t

@@ -517,5 +517,8 b' test -rMULTIREV with --all-files'
   $ hg grep -r "0:2" "unmod" --all-files um
   um:0:unmod
   um:1:unmod
+  $ hg grep -r "0:2" "unmod" --all-files "glob:**/um" # Check that patterns also work
+  um:0:unmod
+  um:1:unmod
   $ cd ..