Added --dir option to `be diff'.

Added --dir option to `be diff'.

Now you can compare two repositories:
  be -d branchA diff -d branchB
or
  branchA$ be diff -d ../branchB
which is helpful for VCSs like bzr that lack cross-branch revid
visibility.  Git users can still use
  branchA$ be diff REVID
where REVID is a commit from any branch in the repo.

This new functionality acts like a BE counterpart to `bzr missing DIR'.