grep: adds allfiles mode
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 behavior is desired for grep.
Support for multiple revisions to be added later.
Differential Revision: https://phab.mercurial-scm.org/D3728