[MERGE] Add support for previewing the result of a merge

[Michael Hudson]

John Arbash Meinel wrote:

> Isn't showing a diff a "show me what you would have done" ?
> I suppose it is outputting a diff to stdout which isn't the same as
> --dry-run.
> 
> I don't know that "-n" was the finally determined short option for it.
> Just that if we do start using this, I think it would be nice to our
> users if all options that fall under "show me what you would have done"
> were unified.

-n is at least somewhat traditional as the short "dry run" option letter 
-- make uses it, for example.

Cheers,
mwh