add reverse iteration for Cmd

[stevengj]

I noticed in #43497 that Cmd supports iteration but not reverse iteration, which is a minor oversight but is easy to correct so why not?

[stevengj]

Should be good to merge?

[KristofferC]

When would you want to do this?

[stevengj]

The basic principle here is that any iterator should support reverse iteration if possible...it's part of being a full-featured iterator. Here, it's a one-line method, so why not?

[ViralBShah]

Are we holding off on the merge here?

[stevengj]

I don't see any reason not to merge if people don't object.

I agree that this has infinitesimal practical impact, but it's a small patch and it just seems like part of fully implementing iteration, which crops up in various places (e.g. last(itr, n) relies on reversibility).