Allow the reviewer to choose the base commit for the pull request Overview (BB-7951)

Workflow:

1. Alice creates pull request with commits A, B, C.
2. Bob reviews the request, leaving myriad comments.
3. Alice addresses the comments via commits D and E, and adds them to the pull.
4. Bob reviews the changes made in D & E and approves.
5. Alice merges the pull.

Currently, to accomplish (4), Bob has to review the changes in D and E individually in the commit view, or aggregated with commits A-C in the overview. In no view, can he examine D & E together.

This could be accomplished be allow the "Base" of the overview to be specified by commit. By default, it is set to the earliest commit of the pull, "A" in our example. At step (4), Bob can set the base to "D". Assuming there were further changes necessary and another round of changes, it could later be updated to "F" and so on. This base is purely a way to restrict the diff view, it does not affect what is being merged.

As a point of reference, reviewboard.org supports this functionality (in a buggy fashion) via their diff revision.

Without this feature, the user can try to ensure that every round of review changes are submitted via a single commit, possibly using git commit --amend to squash together changes. In addition, in the commit view, the comments on the main overview are not visible.

Since an (imperfect) workaround exists, marking as minor, although I believe this is a differentiating feature among code review tools.

Thanks!