adds an allfiles flag that lets you grep on all files in the revision
and not just the one that were modified in that changeset.
This would work on a single revision and get all the files that were
there in that revision. So it's like grepping on a previous state.
Using this with wdir() :: hg grep -r "wdir()" --allfiles is what the
default behaviour is desired for grep.
Support for multiple revisions to be added later