Summary:
Add a new `--only-files-in-revs` option to `hg diff`. This filters the files
that are considered down to the set of files that are touched in either of the
two revisions.
Reviewed By: ryanmce
Differential Revision: D7070102
fbshipit-source-id: b9140cd3a212af9554ef5b76785210c1dbba8ee0
