Feature request: Add option to present a "squash mode" view of multiple stacked commits

[rmloveland]

Prompted by #4, which included this explanation from @jordanlewis :

re presents a review as one set of diffs per commit. If you scroll through that file, you can see that all of the diffs are eventually available for each file - they're just not squashed for you. This allows you to leave comments on diffs from multiple different commits, which facilitates a mode of PR-review where people are trying to PR a sequence of fully-baked commits at once.

The current mode of operation of re isn't set up to facilitate review of PRs that have multiple "fixup" commits stacked onto each other - for that you do want a squash mode of review, which I haven't implemented yet.

Proposed CLI interface (just something to get the discussion started; not sure what's best really):

usage: re [-p owner/repo] [-resume file] [-squash] pr-number

  -squash
        Present the sum of changes to the user as a single diff
  -p string
    	GitHub owner/repo name (default "cockroachdb/cockroach")
   ...