Decompose the events related to reporting the relative appearance of
due_to_rebase chunks so that they are easier to draw inferences from.
Whereas formerly all events were logged as `rebase-delta-percent`, now
three separate events are logged.
- `diff-against-parent`: when the patch range selects a parent, no
due_to_rebase chunks are to be expected, so this is logged as a
separate occurrence.
- `rebase-percent-zero`: when the base of the patch range is another
patch set -- so due_to_rebase chunks are possible -- but none were
included in the diff.
- `rebase-percent-nonzero`: when due_to_rebase chunks have been included
in the diff. Only in this case is the percentage value logged.
Change-Id: I28f639fd5afd45b406a1537f10252fe1b66cd953
688b1f25a2