Add diff and merge capability to `cg_annotate`.

Add diff and merge capability to `cg_annotate`.

And deprecate the use of `cg_diff` and `cg_merge`.

Because `cg_annotate` can do a better job, even annotating source files
when doing diffs in some cases.

The user requests merging by passing multiple cgout files to
`cg_annotate`, and diffing by passing two cgout files to `cg_annotate`
along with `--diff`.